Low emission modular flare stack

ABSTRACT

A low emissions modular flare stack includes a plurality of flare stack burner modules, each including a main fuel source configured to selectively deliver a main fuel stream for dilution by a flow of combustion air, a main igniter configured to cause ignition of the main fuel stream emitted from the main fuel source, a distal flame holder configured to hold a combustion reaction supported by the main fuel stream when the distal flame holder is at or above a predetermined temperature, and a pre-heating apparatus configured to pre-heat the distal flame holder to the predetermined temperature. The low emissions modular flare stack includes a common combustion air source configured to provide combustion air to each of the plurality of flare stack burner modules, and a wall encircling all of the plurality of flare stack burner modules, the wall being configured to laterally contain combustion products corresponding to all of the plurality of flare stack burner modules.

BACKGROUND

Flare stacks are used to burn off vented volatile organic compounds. For example, in an oil refinery, a flare stack may be used to provide emergency burning of volatile compounds, or provide for a safe way to relieve high sudden pressure events of flammable materials. In an oil field, a flare stack may be used to burn off natural gas that is produced as a byproduct of crude oil production. In a landfill, a flare stack may be used to burn off methane released by decomposition processes. Because volatile compounds are considered pollutants and are often flammable, it is generally considered preferable to burn the volatile compounds, rather than to vent the volatile compounds directly to the atmosphere.

In flare stack applications, it can be important to control the height of a flame envelope created by the burner. In some applications, especially those known by the term of art “enclosed flares,” it may be required or desired that the flame not exceed the height of the flare stack itself. By keeping the flame inside the flare stack, safety may be improved. Moreover, aesthetics may be improved sufficiently to avoid complaints about a visible flame.

Enclosed flare stacks or ground flares can be used for burning off unusable waste field gas in a variety of oil and gas production applications, for example. Waste gases may be released during over-pressuring of plant equipment. The waste gases may be transported to a corresponding ground flare. Some ground flares are enclosed. By “enclosed” it is meant that a flame envelope is substantially blocked from view by persons outside a controlled access area.

Flame length may determine a required height, girth, or other dimensions of the ground flare structure. A problem may arise when the flame becomes visible (e.g., is too high). Excessively high flame length may substantially halt operation, and/or may result in fines or be expressed as greater capital cost, increased operating expenses, and/or other remediation expenses.

SUMMARY

According to an embodiment a low emissions modular flare stack may include a plurality of flare stack burner modules, a common combustion air source, and a wall encircling all of the plurality of flare stack burner modules. Each flare stack burner module may include a main fuel source, a main fuel igniter, a distal flame holder and pre-heating apparatus. The main fuel source may be separately valved from all other fuel sources, and may be configured to selectively deliver a main fuel stream for dilution by a flow of combustion air. The main fuel igniter may be configured to cause ignition of the main fuel stream emitted from the main fuel source. The distal flame holder may be separated from the main fuel source and the main fuel igniter by respective non-zero distances, and the distal flame holder may be configured to hold a combustion reaction supported by the main fuel stream when the distal flame holder is at or above a predetermined temperature. The pre-heating apparatus may be configured to pre-heat the distal flame holder to the predetermined temperature.

The common combustion air source is configured to provide combustion air to each of the plurality of flare stack burner modules. The wall is configured to laterally contain combustion fluids corresponding to all of the plurality of flare stack burner modules.

According to an embodiment, a modular flare stack device includes a housing having a combustion air inlet at a base, and a plurality of flare stack burner modules positioned inside the housing. Each flare stack burner module includes an inlet configured to be coupled to a waste fuel supply and to receive the combustion air, a distal flame holder positioned inside the housing, and a main nozzle configured to receive a flow of main fuel from the inlet and to emit a main fuel stream toward the distal flame holder.

According to an embodiment, a method of using a flare stack having a plurality of flare stack burner modules includes outputting toward a distal flame holder of each of the flare stack burner modules a waste gas and a supplemental fuel added to sufficiently raise a heating value of the waste gas plus the supplemental fuel to about 100 BTU per cubic foot or less. The method includes combusting the waste gas and the supplemental fuel substantially within one or more of the plurality of flare stack burner modules.

According to an embodiment, a flare stack burner module includes a pilot fuel nozzle disposed at a distal position along a fuel flow axis, and a plurality of main fuel nozzles disposed at a proximal position along the fuel flow axis. The position of the main fuel nozzles may be referred to as a primary fuel dump plane, the plane lying transverse a combustion air flow axis. The pilot fuel nozzle is configured to support a pilot flame and the plurality of main fuel nozzles are configured to simultaneously support a main flame in contact with the pilot flame. The main flame may, at least once, be ignited by the pilot flame.

According to an embodiment, a flare stack includes a plurality of flare stack burner modules. Each flare stack burner module may include a main fuel source disposed at a proximal position along a flow axis of a corresponding flare stack burner module, a pilot fuel nozzle disposed at an intermediate distance along the flow axis, and a distal flame holder disposed at a distal position along the flow axis. The pilot fuel nozzle may be configured to support a pilot flame to heat the distal flame holder. The main fuel source may be configured to provide main fuel to the distal flame holder after the distal flame holder is at least partially heated. The distal flame holder may be configured to hold at least a portion of a main combustion reaction supported by the main fuel.

According to an embodiment, a method for operating a flare stack burner module having a distal flame holder includes providing heat to the distal flame holder with a pilot flame holder. The distal flame holder and the pilot flame holder may be disposed in a flare stack burner module and in proximity to one another. The method includes introducing main fuel and combustion air to the pilot flame holder and the distal flame holder, and holding at least a portion of a combustion reaction of the main fuel and combustion air at the distal flame holder while the pilot flame holder remains lit.

According to an embodiment, a method for operating a flare stack burner module includes supporting a diffusion flame across a portion of a width of a flare stack burner module volume at a position distal from a flare stack burner module bottom, providing combustion air to the flare stack burner module volume from a location near the flare stack burner module bottom, outputting high pressure main fuel jets from a plurality of main fuel nozzles at the location near the flare stack burner module bottom, mixing the main fuel with the combustion air while the main fuel and combustion air travels from the location near the flare stack burner module bottom to the distal position in the flare stack burner module, and combusting the main fuel by exposing the mixed main fuel and air to the diffusion flame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a flare stack with a distal flame holder, according to an embodiment.

FIG. 2 is a simplified diagram view of a burner system including a perforated flame holder, according to an embodiment.

FIG. 3 is a side sectional diagram of a portion of the perforated flame holder of FIGS. 1 and 2, according to an embodiment.

FIG. 4 is a flow chart showing a method for operating a burner system including the perforated flame holder of FIGS. 1, 2 and 3, according to an embodiment.

FIG. 5 is a diagrammatic side-sectional view of a portion of a flare stack burner module that includes a perforated flame holder substantially as described with reference to FIGS. 2 and 3, according to an embodiment.

FIG. 6 is a diagrammatic side-sectional view of a portion of a flare stack burner module, according to another embodiment.

FIG. 7 is a diagrammatic side-sectional view of a portion of a flare stack burner module that includes a retrofit burner installed in a pre-existing flare stack, according to an embodiment.

FIG. 8 is a view of a low emissions modular flare stack, according to an embodiment.

FIG. 9 is a diagram of an arrangement for each of a plurality of flare stack burner modules of FIG. 8, according to an embodiment.

FIG. 10 is a diagram of a control circuit for use in the control system of FIG. 9, according to an embodiment.

FIG. 11A is a block diagram of a flare stack burner module, according to an embodiment.

FIG. 11B is a block diagram of a flare stack burner module including a bluff body flame holder, according to an embodiment.

FIG. 12A is an illustration of a flare stack burner module, according to an embodiment.

FIG. 12B is an illustration of a flare stack burner module including a distal flame holder, according to an embodiment.

FIG. 13 is a perspective view of a flare stack burner module, according to an embodiment.

FIG. 14 is an illustration of a pilot fuel nozzle in the shape of an H, according to an embodiment.

FIG. 15 is an illustration of a pilot fuel nozzle in the shape of a spiral, according to an embodiment.

FIG. 16 is an illustration of a pilot fuel nozzle in the shape of a hexagon, according to an embodiment.

FIG. 17A is a simplified perspective view of a combustion system, including a reticulated ceramic perforated flame holder, according to an embodiment.

FIG. 17B is a simplified side sectional diagram of a portion of the reticulated ceramic perforated flame holder of FIG. 17A, according to an embodiment.

FIG. 18 is a flow chart showing a method for operating a perforated flame holder flare stack burner module, according to an embodiment.

FIG. 19 is a flow chart showing a method for operating a flare stack burner module, according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here.

FIG. 1 is a diagram of a flare stack 100 with a perforated flame holder 102, according to an embodiment. The flare stack 100 includes a stack structure 104 configured to support a flare. A flare is a combustion reaction that burns off volatile compounds to control pressure in systems such as oil production, oil refining, and other chemical processing systems. A volatile compound source 106 at least intermittently outputs a flow of high vapor pressure flammable compounds. Optionally, a pressure control valve 108 may provide a constant pressure sink to the volatile compound source 106, and determine a constant pressure flow of volatile compounds to a volatile compound nozzle 110. For some systems, this can cause a variable flow rate of volatile compounds to the volatile compound nozzle 110. When volatile compound flow is sufficient, the volatile compound nozzle 110 outputs a stream of volatile compounds. A combustion air source 112, such as a damper or a blower, provides combustion air. The stream of volatile compounds flows and entrains the combustion air to form a volatile compound mixture 114. The perforated flame holder 102 is supported by a perforated flame holder support structure 222 at a position selected to receive the volatile compound mixture 114. As described elsewhere herein, a combustion reaction supported at least partially by the volatile compound mixture 114 can be held by the perforated flame holder 102.

According to an embodiment, a temperature-maintenance fuel nozzle 116 is configured to output a start-up flame or a temperature-maintenance fuel and air mixture 118 (also referenced herein as fuel and oxidant mixture 206) to establish or maintain an operating temperature of the perforated flame holder 102 using fuel from a fuel source 120.

Because flow from the volatile compound source 106 can be intermittent or at least non-steady, the temperature-maintenance fuel nozzle 116 can be configured to cooperate with the fuel source 120 to provide a relatively high fuel and air mixture 118 flow rate when the volatile compound mixture 114 flow rate is low and provide a relatively low or zero fuel and air mixture 118 flow rate when the volatile compound mixture 114 flow rate is high.

According to an embodiment, a controller 122 is operatively coupled to a flow sensor 124 configured to measure flow of volatile compounds from the volatile compound source 106. The controller 122 can use digital logic to determine a corresponding flow rate appropriate for the fuel and air mixture 118, and control a fuel flow valve 126 to provide a selected flow rate of the fuel from the fuel source 120 to the temperature-maintenance fuel nozzle 116.

The temperature-maintenance fuel nozzle 116 can include a fuel riser 128 and an ignition source 129 configured to ignite a start-up flame near the temperature-maintenance fuel nozzle 116. The ignition source 129 can include a hot surface igniter, a spark-discharge igniter, or a pilot flame, for example. Additionally, or alternatively, the ignition source 129 may include a flame holder operable to hold a flame at a location proximate to the fuel riser 128. The flame holder may be configured to be actuated to selectively hold a flame at the location proximate to the fuel source 120 or to allow the fuel from the fuel source 120 to travel to the perforated flame holder 102 for combustion. In such an embodiment, the ignition source 129 may additionally include a separate igniter, or alternatively the fuel riser 128 may be manually ignited at start-up.

According to an embodiment, when the ignition source 129 or actuatable flame holder is enabled, a start-up flame is supported between the temperature-maintenance fuel nozzle 116 and the perforated flame holder 102. When the ignition source 129 or actuatable flame holder is not enabled, the temperature-maintenance fuel nozzle 116 outputs a flow of the fuel and air mixture 118 to the perforated flame holder 102 for combustion in the perforated flame holder 102. The controller 122 can be operatively coupled to the ignition source 129 to determine whether a start-up flame is supported or whether the fuel and air mixture 118 is delivered to the perforated flame holder 102 for combustion.

According to an embodiment, the temperature-maintenance fuel nozzle 116 can be configured to add a relatively high BTU-content fuel, such as propane or natural gas, to a relatively low BTU-content fuel from the volatile compound nozzle 110. For continuous flow operations, the “temperature maintenance” performed by the temperature-maintenance fuel nozzle 116 may consist essentially of increasing the BTU content of the combustible materials (fuel plus volatile compound) delivered to the perforated flame holder 102.

In one experiment, it was found that use of the perforated flame holder 102 could reduce the necessary BTU content of methane fuel plus the volatile compound mixture 114 from 300 BTUs per cubic foot to below 100 BTUs per cubic foot while maintaining steady combustion, compared to burning the volatile compound in a conventional flame. The capabilities of the perforated flame holder 102 can thus be used to advantage in many waste gas burn-off applications, whether or not in a contained flame flare stack, and can result in significant fuel cost savings.

During times when substantially no volatile compounds are output from the volatile compound source 106, the controller 122 can cause the flare stack 100 to operate in a “cold standby” state, where minimal or no fuel from the fuel source 120 is consumed, and the fuel flow valve 126 is maintained in an “off” position. Optionally, the flare stack 100 can include a volatile compound flow valve 130 operatively coupled to the controller 122. The controller 122 can hold the volatile compound flow valve 130 in an off state whenever the flare stack 100 is in a cold standby state.

When an imminent volatile compound flow is detected (e.g., by a pressure sensor (not shown) or the volatile compound flow sensor 124), the controller 122 can convert the flare stack 100 to a “warm standby” state, wherein the fuel flow valve 126 is opened sufficiently, and the ignition source 129 enabled to support a start-up flame. The flare stack 100 can change from a warm standby state to a “hot standby” state when the temperature of the space between the volatile compound nozzle 110 and the perforated flame holder 102 is warmed by the start-up flame to a sufficiently hot temperature to ensure complete combustion of the volatile compound. In the hot standby state, the flare stack 100 can operate as a normal flare stack with volatile compound flaring occurring in a conventional flame below the perforated flame holder 102. In some cases, the volatile compound is itself a fuel of sufficient heating value to provide a continual flame without any additional or supporting fuel. In other cases, a supplemental fuel is required to raise the heat value of the (supplemental) fuel plus volatile compound. In still other cases, it is more proper to call the fuel an ignition fuel inasmuch as the volatile compound has sufficient heating value to maintain the combustion reaction, but for reasons of safety or convenience, it is preferable to have a fuel of known composition and pressure available as an ignition source. Unless noted, the term “fuel” may function in any of these senses.

When the perforated flame holder 102 is warmed to a start-up temperature, the controller 122 can disable the ignition source 129 to lift from the start-up flame location and cause the fuel/air mixture 118 to impinge on the perforated flame holder 102, wherein combustion is held. Simultaneously, with no ignition by the start-up flame, the volatile compound/air mixture 114 travels to the perforated flame holder 102 wherein the volatile compound is combusted.

When the volatile compound flow rate is sufficiently high to maintain combustion in the perforated flame holder 102, the controller 122 can reduce the fuel flow rate or stop fuel flow using the fuel flow valve 126. Optionally, the controller 122 can include a proportional controller configured to maintain a fuel and air mixture flow rate that is inversely proportional to the volatile compound mixture 114 flow rate.

Optionally, the controller 122 can be operatively coupled to control the combustion air source 112. Optionally, the controller 122 can be operatively coupled to a sensor 132 configured to sense combustion, temperature, or other parameter related to performance of the flare stack 100.

According to an embodiment, the distal flame holder 102 includes one or more refractory bluff bodies. In an embodiment, at least a portion of the one or more refractory bluff bodies includes one or more perforated tiles (also referenced, below, as perforated flame holders 102). In an embodiment, at least a portion of the one or more refractory bluff bodies includes one or more solid tiles. In an embodiment, the one or more perforated tiles are configured to support the combustion reaction of the fuel and the oxidant upstream, downstream, between, and within the perforated flame holders 102. In an embodiment, one or more solid bluff bodies are configured to support the combustion reaction of the fuel and the oxidant upstream, downstream, and between the solid bluff bodies. According to embodiments, when the distal flame holder 102 is heated to an operating temperature corresponding to an auto-ignition temperature of the fuel, the perforated and/or solid bluff bodies recycle heat released during combustion to raise the temperature of incoming fuel and combustion air sufficiently to ensure ignition. A description of a perforated flame holder 102 follows, with reference to FIGS. 2-4.

FIG. 2 is a simplified diagram of a burner system 200 including a perforated flame holder 102 configured to hold a combustion reaction, according to an embodiment that may operate in a method analogous to a flare stack burner module. As used herein, the terms distal flame holder, bluff body flame holder, perforated reaction holder, perforated flame holder, porous flame holder, porous reaction holder, duplex, duplex tile, and at least some embodiments of a distal flame holder shall be considered synonymous unless further definition is provided.

Experiments performed by the inventors have shown that perforated flame holders 102 described herein can support very clean combustion. Specifically, in experimental use of burner systems 200 ranging from pilot scale to full scale, output of oxides of nitrogen (NOx) was measured to range from low single digit parts per million (ppm) down to undetectable (less than 1 ppm) concentration of NOx at the stack. These remarkable results were measured at 3% (dry) oxygen (O₂) concentration with undetectable carbon monoxide (CO) at stack temperatures typical of industrial furnace applications (1400-1600° F.). Moreover, these results did not require any extraordinary measures such as selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), water/steam injection, external flue gas recirculation (FGR), or other heroic extremes that may be required for conventional burners to even approach such clean combustion.

According to embodiments, the burner system 200 includes a fuel and oxidant source 202 disposed to output fuel and oxidant into a combustion volume 204 to form a fuel and oxidant mixture 206. As used herein, the terms fuel and oxidant mixture and fuel stream may be used interchangeably and considered synonymous depending on the context, unless further definition is provided. As used herein, the terms combustion volume, combustion chamber, furnace volume, and the like shall be considered synonymous unless further definition is provided. The perforated flame holder 102 is disposed in the combustion volume 204 and positioned to receive the fuel and oxidant mixture 206.

FIG. 3 is a side sectional diagram 300 of a portion of the perforated flame holder 102 of FIGS. 1 and 2, according to an embodiment. Referring to FIGS. 2 and 3, the perforated flame holder 102 includes a perforated flame holder body 208 defining a plurality of perforations 210 aligned to receive the fuel and oxidant mixture 206 from the fuel and oxidant source 202. As used herein, the terms perforation, pore, aperture, elongated aperture, and the like, in the context of the perforated flame holder 102, shall be considered synonymous unless further definition is provided. The perforations 210 are configured to collectively hold a combustion reaction 302 supported by the fuel and oxidant mixture 206.

The fuel can include hydrogen, a hydrocarbon gas, a vaporized hydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered or pulverized solid. The fuel can be a single species or can include a mixture of gas(es), vapor(s), atomized liquid(s), and/or pulverized solid(s). For example, in a process heater application the fuel can include fuel gas or byproducts from the process that include carbon monoxide (CO), hydrogen (H₂), and methane (CH₄). In another application, the fuel can include natural gas (mostly CH₄) or propane (C₃H₈). In another application, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuel applications and flexible fuel applications are similarly contemplated by the inventors. The oxidant can include oxygen carried by air, flue gas, and/or can include another oxidant, either pure or carried by a carrier gas. The terms oxidant and oxidizer shall be considered synonymous herein.

According to an embodiment, the perforated flame holder body 208 can be bounded by an input face 212 disposed to receive the fuel and oxidant mixture 206, an output face 214 facing away from the fuel and oxidant source 202, and a peripheral surface 216 defining a lateral extent of the perforated flame holder 102. The plurality of perforations 210 which are defined by the perforated flame holder body 208 extend from the input face 212 to the output face 214. The plurality of perforations 210 can receive the fuel and oxidant mixture 206 at the input face 212. The fuel and oxidant mixture 206 can then combust in or near the plurality of perforations 210 and combustion products can exit the plurality of perforations 210 at or near the output face 214.

According to an embodiment, the perforated flame holder 102 is configured to hold a majority of the combustion reaction 302 within the perforations 210. For example, on a steady-state basis, more than half the molecules of fuel output into the combustion volume 204 by the fuel and oxidant source 202 may be converted to combustion products between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, more than half of the heat or thermal energy output by the combustion reaction 302 may be output between the input face 212 and the output face 214 of the perforated flame holder 102. As used herein, the terms heat, heat energy, and thermal energy shall be considered synonymous unless further definition is provided. As used above, heat energy and thermal energy refer generally to the released chemical energy initially held by reactants during the combustion reaction 302. As used elsewhere herein, heat, heat energy and thermal energy correspond to a detectable temperature rise undergone by real bodies characterized by heat capacities. Under nominal operating conditions, the perforations 210 can be configured to collectively hold at least 80% of the combustion reaction 302 between the input face 212 and the output face 214 of the perforated flame holder 102. In some experiments, the inventors produced a combustion reaction 302 that was apparently wholly contained in the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. According to an alternative interpretation, the perforated flame holder 102 can support combustion between the input face 212 and output face 214 when combustion is “time-averaged.” For example, during transients, such as before the perforated flame holder 102 is fully heated, or if too high a (cooling) load is placed on the system, the combustion may travel somewhat downstream from the output face 214 of the perforated flame holder 102. Alternatively, if the cooling load is relatively low and/or the furnace temperature reaches a high level, the combustion may travel somewhat upstream of the input face 212 of the perforated flame holder 102.

While a “flame” is described in a manner intended for ease of description, it should be understood that in some instances, no visible flame is present. Combustion occurs primarily within the perforations 210, but the “glow” of combustion heat is dominated by a visible glow of the perforated flame holder 102 itself. In other instances, the inventors have noted transient “huffing” or “flashback” wherein a visible flame momentarily ignites in a region lying between the input face 212 of the perforated flame holder 102 and a fuel nozzle 218, within the dilution region D_(D). Such transient huffing or flashback is generally short in duration such that, on a time-averaged basis, a majority of combustion occurs within the perforations 210 of the perforated flame holder 102, between the input face 212 and the output face 214. In still other instances, the inventors have noted apparent combustion occurring downstream from the output face 214 of the perforated flame holder 102, but still a majority of combustion occurred within the perforated flame holder 102 as evidenced by continued visible glow from the perforated flame holder 102 that was observed.

The perforated flame holder 102 can be configured to receive heat from the combustion reaction 302 and output a portion of the received heat as thermal radiation 304 to heat-receiving structures (e.g., furnace walls and/or radiant section working fluid tubes) in or adjacent to the combustion volume 204. As used herein, terms such as radiation, thermal radiation, radiant heat, heat radiation, etc. are to be construed as being substantially synonymous, unless further definition is provided. Specifically, such terms refer to blackbody-type radiation of electromagnetic energy, primarily at infrared wavelengths, but also at visible wavelengths owing to elevated temperature of the perforated flame holder body 208.

Referring especially to FIG. 3, the perforated flame holder 102 outputs another portion of the received heat to the fuel and oxidant mixture 206 received at the input face 212 of the perforated flame holder 102. The perforated flame holder body 208 may receive heat from the combustion reaction 302 at least in heat receiving regions 306 of perforation walls 308. Experimental evidence has suggested to the inventors that the position of the heat receiving regions 306, or at least the position corresponding to a maximum rate of receipt of heat, can vary along the length of the perforation walls 308. In some experiments, the location of maximum receipt of heat was apparently between ⅓ and ½ of the distance from the input face 212 to the output face 214 (i.e., somewhat nearer to the input face 212 than to the output face 214). The inventors contemplate that the heat receiving regions 306 may lie nearer to the output face 214 of the perforated flame holder 102 under other conditions. Most probably, there is no clearly defined edge of the heat receiving regions 306 (or for that matter, heat output regions 310, described below). For ease of understanding, the heat receiving regions 306 and the heat output regions 310 will be described as particular regions 306, 310.

The perforated flame holder body 208 can be characterized by a heat capacity. The perforated flame holder body 208 may hold thermal energy from the combustion reaction 302 in an amount corresponding to the heat capacity multiplied by temperature rise, and transfer the thermal energy from the heat receiving regions 306 to heat output regions 310 of the perforation walls 308. Generally, the heat output regions 310 are nearer to the input face 212 than are the heat receiving regions 306. According to one interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via thermal radiation, depicted graphically as 304. According to another interpretation, the perforated flame holder body 208 can transfer heat from the heat receiving regions 306 to the heat output regions 310 via heat conduction along heat conduction paths 312. The inventors contemplate that multiple heat transfer mechanisms including conduction, radiation, and possibly convection may be operative in transferring heat from the heat receiving regions 306 to the heat output regions 310. In this way, the perforated flame holder 102 may act as a heat source to maintain the combustion reaction 302, even under conditions where a combustion reaction 302 would not be stable when supported from a conventional flame holder.

The inventors believe that the perforated flame holder 102 causes the combustion reaction 302 to begin within thermal boundary layers 314 formed adjacent to walls 308 of the perforations 210. Insofar as combustion is generally understood to include a large number of individual reactions, and since a large portion of combustion energy is released within the perforated flame holder 102, it is apparent that at least a majority of the individual reactions occur within the perforated flame holder 102. As the relatively cool fuel and oxidant mixture 206 approaches the input face 212, the flow is split into portions that respectively travel through individual perforations 210. The hot perforated flame holder body 208 transfers heat to the fluid, notably within thermal boundary layers 314 that progressively thicken as more and more heat is transferred to the incoming fuel and oxidant mixture 206. After reaching a combustion temperature (e.g., the auto-ignition temperature of the fuel), the reactants continue to flow while a chemical ignition delay time elapses, over which time the combustion reaction 302 occurs. Accordingly, the combustion reaction 302 is shown as occurring within the thermal boundary layers 314. As flow progresses, the thermal boundary layers 314 merge at a merger point 316. Ideally, the merger point 316 lies between the input face 212 and the output face 214 that define the ends of the perforations 210. At some position along the length of a perforation 210, the combustion reaction 302 outputs more heat to the perforated flame holder body 208 than it receives from the perforated flame holder body 208. The heat is received at the heat receiving region 306, is held by the perforated flame holder body 208, and is transported to the heat output region 310 nearer to the input face 212, where the heat is transferred into the cool reactants (and any included diluent) to bring the reactants to the ignition temperature.

In an embodiment, each of the perforations 210 is characterized by a length L defined as a reaction fluid propagation path length between the input face 212 and the output face 214 of the perforated flame holder 102. As used herein, the term reaction fluid refers to matter that travels through a perforation 210. Near the input face 212, the reaction fluid includes the fuel and oxidant mixture 206 (optionally including nitrogen, flue gas, and/or other “non-reactive” species). Within a combustion reaction region, the reaction fluid may include plasma associated with the combustion reaction 302, molecules of reactants and their constituent parts, any non-reactive species, reaction intermediates (including transition states), and reaction products. Near the output face 214, the reaction fluid may include reaction products and byproducts, non-reactive gas, and excess oxidant.

The plurality of perforations 210 can be each characterized by a transverse dimension D between opposing perforation walls 308. The inventors have found that stable combustion can be maintained in the perforated flame holder 102 if the length L of each perforation 210 is at least four times the transverse dimension D of the perforation. In other embodiments, the length L can be greater than six times the transverse dimension D. For example, experiments have been run where L is at least eight, at least twelve, at least sixteen, and at least twenty-four times the transverse dimension D. Preferably, the length L is sufficiently long for thermal boundary layers 314 to form adjacent to the perforation walls 308 in a reaction fluid flowing through the perforations 210 to converge at merger points 316 within the perforations 210 between the input face 212 and the output face 214 of the perforated flame holder 102. In experiments, the inventors have found L/D ratios between 12 and 48 to work well (i.e., produce low NOx, produce low CO, and maintain stable combustion).

The perforated flame holder body 208 can be configured to convey heat between adjacent perforations 210. The heat conveyed between adjacent perforations 210 can be selected to cause heat output from the combustion reaction portion 302 in a first perforation 210 to supply heat to stabilize a combustion reaction portion 302 in an adjacent perforation 210.

Referring especially to FIG. 2, the fuel and oxidant source 202 can further include the fuel nozzle 218, configured to output the fuel, and an oxidant source 220 configured to output a fluid including the oxidant. For example, the fuel nozzle 218 can be configured to output pure fuel. The oxidant source 220 can be configured to output combustion air carrying oxygen, and optionally, flue gas.

The perforated flame holder 102 can be held by a perforated flame holder support structure 222 configured to hold the perforated flame holder 102 at a dilution distance D_(D) away from the fuel nozzle 218. The fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant to form the fuel and oxidant mixture 206 as the fuel jet and the oxidant travel along a path to the perforated flame holder 102 through the dilution distance D_(D) between the fuel nozzle 218 and the perforated flame holder 102. Additionally or alternatively (particularly when a blower is used to deliver oxidant contained in combustion air), the oxidant or combustion air source 220 can be configured to entrain the fuel and the fuel and the oxidant travel through the dilution distance D_(D). In some embodiments, a flue gas recirculation path 224 can be provided. Additionally or alternatively, the fuel nozzle 218 can be configured to emit a fuel jet selected to entrain the oxidant and to entrain flue gas as the fuel jet travels through the dilution distance D_(D) between the fuel nozzle 218 and the input face 212 of the perforated flame holder 102.

The fuel nozzle 218 can be configured to emit the fuel through one or more fuel orifices 226 having an inside diameter dimension that is referred to as “nozzle diameter.” The perforated flame holder support structure 222 can support the perforated flame holder 102 to receive the fuel and oxidant mixture 206 at the distance D_(D) away from the fuel nozzle 218 greater than 20 times the nozzle diameter. In another embodiment, the perforated flame holder 102 is disposed to receive the fuel and oxidant mixture 206 at the distance D_(D) away from the fuel nozzle 218 between 100 times and 1100 times the nozzle diameter. Preferably, the perforated flame holder support structure 222 is configured to hold the perforated flame holder 102 at a distance about 200 times or more of the nozzle diameter away from the fuel nozzle 218. When the fuel and oxidant mixture 206 travels about 200 times the nozzle diameter or more, the mixture 206 is sufficiently homogenized to cause the combustion reaction 302 to produce minimal NOx.

The fuel and oxidant source 202 can alternatively include a premix fuel and oxidant source, according to an embodiment. A premix fuel and oxidant source can include a premix chamber (not shown), a fuel nozzle configured to output fuel into the premix chamber, and an oxidant (e.g., combustion air) channel configured to output the oxidant into the premix chamber. A flame arrestor can be disposed between the premix fuel and oxidant source and the perforated flame holder 102 and be configured to prevent flame flashback into the premix fuel and oxidant source.

The oxidant source 220, whether configured for entrainment in the combustion volume 204 or for premixing, can include a blower configured to force the oxidant through the fuel and oxidant source 202.

The perforated flame holder support structure 222 can be configured to support the perforated flame holder 102 from a bottom or wall (not shown) of the combustion volume 204, for example. In another embodiment, the perforated flame holder support structure 222 supports the perforated flame holder 102 from the fuel and oxidant source 202. Alternatively, the perforated flame holder support structure 222 can suspend the perforated flame holder 102 from an overhead structure (such as a flue, in the case of an up-fired system). The perforated flame holder support structure 222 can support the perforated flame holder 102 in various orientations and directions.

The perforated flame holder 102 can include a single perforated flame holder body 208. In another embodiment, the perforated flame holder 102 can include a plurality of adjacent perforated flame holder sections that collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 222 can be configured to support the plurality of perforated flame holder sections. The perforated flame holder support structure 222 can include a metal superalloy, a cementatious, and/or ceramic refractory material. In an embodiment, the plurality of adjacent perforated flame holder sections can be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least twice a thickness dimension T between the input face 212 and the output face 214. In another embodiment, the perforated flame holder 102 can have a width dimension W between opposite sides of the peripheral surface 216 at least three times, at least six times, or at least nine times the thickness dimension T between the input face 212 and the output face 214 of the perforated flame holder 102.

In an embodiment, the perforated flame holder 102 can have a width dimension W less than a width of the combustion volume 204. This can allow the flue gas recirculation path 224 from above to below the perforated flame holder 102 to lie between the peripheral surface 216 of the perforated flame holder 102 and the combustion volume wall (not shown).

Referring again to both FIGS. 2 and 3, the perforations 210 can be of various shapes. In an embodiment, the perforations 210 can include elongated squares, each having a transverse dimension D between opposing sides of the squares. In another embodiment, the perforations 210 can include elongated hexagons, each having a transverse dimension D between opposing sides of the hexagons. In yet another embodiment, the perforations 210 can include hollow cylinders, each having a transverse dimension D corresponding to a diameter of the cylinder. In another embodiment, the perforations 210 can include truncated cones or truncated pyramids (e.g., frustums), each having a transverse dimension D radially symmetric relative to a length axis that extends from the input face 212 to the output face 214. In some embodiments, the perforations 210 can each have a lateral dimension D equal to or greater than a quenching distance of a flame based on standard reference conditions. Alternatively, the perforations 210 may have lateral dimension D less than a standard reference quenching distance.

In one range of embodiments, each of the plurality of perforations 210 has a lateral dimension D between 0.05 inch and 1.0 inch. Preferably, each of the plurality of perforations 210 has a lateral dimension D between 0.1 inch and 0.5 inch. For example, the plurality of perforations 210 can each have a lateral dimension D of about 0.2 to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as the total volume of all perforations 210 in a section of the perforated flame holder 102 divided by a total volume of the perforated flame holder 102 including the perforated flame holder body 208 and the perforations 210. The perforated flame holder 102 should have a void fraction between 0.10 and 0.90. In an embodiment, the perforated flame holder 102 can have a void fraction between 0.30 and 0.80. In another embodiment, the perforated flame holder 102 can have a void fraction of about 0.70. Using a void fraction of about 0.70 was found to be especially effective for producing very low NOx.

The perforated flame holder 102 can be formed from a fiber reinforced cast refractory material and/or a refractory material such as an aluminum silicate material. For example, the perforated flame holder 102 can be formed to include mullite or cordierite. Additionally, or alternatively, the perforated flame holder body 208 can include a metal superalloy such as Inconel or Hastelloy. The perforated flame holder body 208 can define a honeycomb. Honeycomb is an industrial term of art that need not strictly refer to a hexagonal cross section and most usually includes cells of square cross section. Honeycombs of other cross-sectional areas are also known.

The inventors have found that the perforated flame holder 102 can be formed from VERSAGRID® ceramic honeycomb, available from Applied Ceramics, Inc. of Doraville, S.C.

The perforations 210 can be parallel to one another and normal to the input and the output faces 212, 214. In another embodiment, the perforations 210 can be parallel to one another and formed at an angle relative to the input and the output faces 212, 214. In another embodiment, the perforations 210 can be non-parallel to one another. In another embodiment, the perforations 210 can be non-parallel to one another and non-intersecting. In another embodiment, the perforations 210 can be intersecting. The perforated flame holder body 208 can be one piece or can be formed from a plurality of sections.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from reticulated ceramic material. The term “reticulated” refers to a netlike structure. Reticulated ceramic material is often made by dissolving a slurry into a sponge of specified porosity, allowing the slurry to harden, and burning away the sponge and curing the ceramic.

In another embodiment, which is not necessarily preferred, the perforated flame holder 102 may be formed from a ceramic material that has been punched, bored or cast to create channels.

In another embodiment, the perforated flame holder 102 can include a plurality of tubes or pipes bundled together. The plurality of perforations 210 can include hollow cylinders and can optionally also include interstitial spaces between the bundled tubes. In an embodiment, the plurality of tubes can include ceramic tubes. Refractory cement can be included between the tubes and configured to adhere the tubes together. In another embodiment, the plurality of tubes can include metal (e.g., superalloy) tubes. The plurality of tubes can be held together by a metal tension member circumferential to the plurality of tubes and arranged to hold the plurality of tubes together. The metal tension member can include stainless steel, a superalloy metal wire, and/or a superalloy metal band.

The perforated flame holder body 208 can alternatively include stacked perforated sheets of material, each sheet having openings that connect with openings of subjacent and superjacent sheets. The perforated sheets can include perforated metal sheets, ceramic sheets and/or expanded sheets. In another embodiment, the perforated flame holder body 208 can include discontinuous packing bodies such that the perforations 210 are formed in the interstitial spaces between the discontinuous packing bodies. In one example, the discontinuous packing bodies include structured packing shapes. In another example, the discontinuous packing bodies include random packing shapes. For example, the discontinuous packing bodies can include ceramic Raschig ring, ceramic Berl saddles, ceramic Intalox saddles, and/or metal rings or other shapes (e.g., Super Raschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systems including the perforated flame holder 102 provide such clean combustion.

According to an embodiment, the perforated flame holder 102 may act as a heat source to maintain a combustion reaction 302 even under conditions where a combustion reaction would not be stable when supported by a conventional flame holder. This capability can be leveraged to support combustion using a leaner fuel-to-oxidant mixture than is typically feasible. Thus, according to an embodiment, at the point where the fuel stream 206 contacts the input face 212 of the perforated flame holder 102, an average fuel-to-oxidant ratio of the fuel stream 206 is below a (conventional) lower combustion limit of the fuel component of the fuel stream 206—lower combustion limit defines the lowest concentration of fuel at which a fuel and oxidant mixture 206 will burn when exposed to a momentary ignition source under normal atmospheric pressure and an ambient temperature of 25° C. (77° F.).

The perforated flame holder 102 and systems including the perforated flame holder 102 described herein were found to provide substantially complete combustion of CO (single digit ppm down to undetectable, depending on experimental conditions), while supporting low NOx. According to one interpretation, such a performance can be achieved due to a sufficient mixing used to lower peak flame temperatures (among other strategies). Flame temperatures tend to peak under slightly rich conditions, which can be evident in any diffusion flame that is insufficiently mixed. By sufficiently mixing, a homogenous and slightly lean mixture can be achieved prior to combustion. This combination can result in reduced flame temperatures, and thus reduced NOx formation. In one embodiment, “slightly lean” may refer to 3% O₂, i.e., an equivalence ratio of ˜0.87. Use of even leaner mixtures is possible, but may result in elevated levels of O₂. Moreover, the inventors believe perforation walls 308 may act as a heat sink for the combustion fluid. This effect may alternatively or additionally reduce combustion temperatures and lower NOx.

According to another interpretation, production of NOx can be reduced if the combustion reaction 302 occurs over a very short duration of time. Rapid combustion causes the reactants (including oxygen and entrained nitrogen) to be exposed to NOx-formation temperature for a time too short for NOx formation kinetics to cause significant production of NOx. The time required for the reactants to pass through the perforated flame holder 102 is very short compared to a conventional flame. The low NOx production associated with perforated flame holder 102 combustion may thus be related to the short duration of time required for the reactants (and entrained nitrogen) to pass through the perforated flame holder 102.

FIG. 4 is a flow chart showing a method 400 for operating a burner system including the perforated flame holder (102) shown and described herein. To operate a burner system including a perforated flame holder, the perforated flame holder is first heated to a temperature sufficient to maintain combustion of the fuel and oxidant mixture.

According to a simplified description, the method 400 begins with step 402, wherein the perforated flame holder is preheated to a start-up temperature, T_(s). After the perforated flame holder is raised to the start-up temperature, the method proceeds to step 404, wherein the fuel and oxidant are provided to the perforated flame holder and combustion is held by the perforated flame holder.

According to a more detailed description, step 402 begins with step 406, wherein start-up energy is provided at the perforated flame holder. Simultaneously or following providing start-up energy, a decision step 408 determines whether the temperature T of the perforated flame holder is at or above the start-up temperature, T. As long as the temperature of the perforated flame holder is below its start-up temperature, the method loops between steps 406 and 408 within the preheat step 402. In decision step 408, if the temperature T of at least a predetermined portion of the perforated flame holder is greater than or equal to the start-up temperature, the method 400 proceeds to overall step 404, wherein fuel and oxidant is supplied to and combustion is held by the perforated flame holder.

Step 404 may be broken down into several discrete steps, at least some of which may occur simultaneously.

Proceeding from decision step 408, a fuel and oxidant mixture is provided to the perforated flame holder, as shown in step 410. The fuel and oxidant may be provided by a fuel and oxidant source that includes a separate fuel nozzle and oxidant (e.g., combustion air) source, for example. In this approach, the fuel and oxidant are output in one or more directions selected to cause the fuel and oxidant mixture to be received by the input face of the perforated flame holder. The fuel may entrain the combustion air (or alternatively, the combustion air may dilute the fuel) to provide a fuel and oxidant mixture at the input face of the perforated flame holder at a fuel dilution selected for a stable combustion reaction that can be held within the perforations of the perforated flame holder.

Proceeding to step 412, the combustion reaction is held by the perforated flame holder.

In step 414, heat may be output from the perforated flame holder. The heat output from the perforated flame holder may be used to power an industrial process, heat a working fluid, generate electricity, or provide motive power, for example.

In optional step 416, the presence of combustion may be sensed. Various sensing approaches have been used and are contemplated by the inventors. Generally, combustion held by the perforated flame holder is very stable and no unusual sensing requirement is placed on the system. Combustion sensing may be performed using an infrared sensor, a video sensor, an ultraviolet sensor, a charged species sensor, thermocouple, thermopile, flame rod, and/or other combustion sensing apparatuses. In an additional or alternative variant of step 416, a pilot flame or other ignition source may be provided to cause ignition of the fuel and oxidant mixture in the event combustion is lost at the perforated flame holder.

Proceeding to decision step 418, if combustion is sensed not to be stable, the method 400 may exit to step 424, wherein an error procedure is executed. For example, the error procedure may include turning off fuel flow, re-executing the pre-heating step 402, outputting an alarm signal, igniting a stand-by combustion system, or other steps. If, in decision step 418, combustion in the perforated flame holder is determined to be stable, the method 400 proceeds to decision step 420, wherein it is determined if combustion parameters should be changed. If no combustion parameters are to be changed, the method loops (within step 404) back to step 410, and the combustion process continues. If a change in combustion parameters is indicated, the method 400 proceeds to step 422, wherein the combustion parameter change is executed. After changing the combustion parameter(s), the method loops (within step 404) back to step 410, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if a change in heat demand is encountered. For example, if less heat is required (e.g., due to decreased electricity demand, decreased motive power requirement, or lower industrial process throughput), the fuel and oxidant flow rate may be decreased in step 422. Conversely, if heat demand is increased, then fuel and oxidant flow may be increased. Additionally, or alternatively, if the combustion system is in a start-up mode, then fuel and oxidant flow may be gradually increased to the perforated flame holder over one or more iterations of the loop within step 404.

Referring again to FIG. 2, the burner system 200 includes a heater 228 operatively coupled to the perforated flame holder 102. As described in conjunction with FIGS. 3 and 4, the perforated flame holder 102 operates by outputting heat to the incoming fuel and oxidant mixture 206. After combustion is established, this heat is provided by the combustion reaction 302; but before combustion is established, the heat is provided by the heater 228.

Various heating apparatuses have been used and are contemplated by the inventors. In some embodiments, the heater 228 can include a flame holder configured to support a flame disposed to heat the perforated flame holder 102. The fuel and oxidant source 202 can include a fuel nozzle 218 configured to emit a fuel stream 206 and an oxidant source 220 configured to output oxidant (e.g., combustion air) adjacent to the fuel stream 206. The fuel nozzle 218 and the oxidant source 220 can be configured to output the fuel stream 206 to be progressively diluted by the oxidant (e.g., combustion air). The perforated flame holder 102 can be disposed to receive a diluted fuel and oxidant mixture 206 that supports a combustion reaction 302 that is stabilized by the perforated flame holder 102 when the perforated flame holder 102 is at an operating temperature. A start-up flame holder, in contrast, can be configured to support a start-up flame at a location corresponding to a relatively unmixed fuel and oxidant mixture that is stable without stabilization provided by the heated perforated flame holder 102.

The burner system 200 can further include a controller 230 operatively coupled to the heater 228 and to a data interface 232. For example, the controller 230 can be configured to control a start-up flame holder actuator configured to cause the start-up flame holder to hold the start-up flame when the perforated flame holder 102 needs to be pre-heated and to not hold the start-up flame when the perforated flame holder 102 is at an operating temperature (e.g., when T≥T_(s)).

Various approaches for actuating a start-up flame are contemplated. In one embodiment, the start-up flame holder includes a mechanically-actuated bluff body configured to be actuated to intercept the fuel and oxidant mixture 206 to cause heat-recycling and/or stabilizing vortices and thereby hold a start-up flame; or to be actuated to not intercept the fuel and oxidant mixture 206 to cause the fuel and oxidant mixture 206 to proceed to the perforated flame holder 102. In another embodiment, a fuel control valve, blower, and/or damper may be used to select a fuel and oxidant mixture flow rate that is sufficiently low for a start-up flame to be jet-stabilized; and upon reaching a perforated flame holder 102 operating temperature, the flow rate may be increased to “blow out” the start-up flame. In another embodiment, the heater 228 may include an electrical power supply operatively coupled to the controller 230 and configured to apply an electrical charge or voltage to the fuel and oxidant mixture 206. An electrically conductive start-up flame holder may be selectively coupled to a voltage ground or other voltage selected to attract the electrical charge in the fuel and oxidant mixture 206. The attraction of the electrical charge was found by the inventors to cause a start-up flame to be held by the electrically conductive start-up flame holder.

In another embodiment, the heater 228 may include an electrical resistance heater configured to output heat to the perforated flame holder 102 and/or to the fuel and oxidant mixture 206. The electrical resistance heater can be configured to heat up the perforated flame holder 102 to an operating temperature. The heater 228 can further include a power supply and a switch operable, under control of the controller 230, to selectively couple the power supply to the electrical resistance heater.

An electrical resistance heater 228 can be formed in various ways. For example, the electrical resistance heater 228 can be formed from KANTHAL® wire (available from Sandvik Materials Technology division of Sandvik AB of Hallstaham mar, Sweden) threaded through at least a portion of the perforations 210 defined by the perforated flame holder body 208. Alternatively, the heater 228 can include an inductive heater, a high-energy beam heater (e.g. microwave or laser), a frictional heater, electro-resistive ceramic coatings, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, the heater 228 can include an electrical discharge igniter or hot surface igniter configured to output a pulsed ignition to the oxidant and the fuel. Additionally, or alternatively, a start-up apparatus can include a pilot flame apparatus disposed to ignite the fuel and oxidant mixture 206 that would otherwise enter the perforated flame holder 102. The electrical discharge igniter, hot surface igniter, and/or pilot flame apparatus can be operatively coupled to the controller 230, which can cause the electrical discharge igniter or pilot flame apparatus to maintain combustion of the fuel and oxidant mixture 206 in or upstream from the perforated flame holder 102 before the perforated flame holder 102 is heated sufficiently to maintain combustion.

The burner system 200 can further include a sensor 234 operatively coupled to the control circuit 230. The sensor 234 can include a heat sensor configured to detect infrared radiation or a temperature of the perforated flame holder 102. The control circuit 230 can be configured to control the heating apparatus 228 responsive to input from the sensor 234. Optionally, a fuel control valve 236 can be operatively coupled to the controller 230 and configured to control a flow of fuel to the fuel and oxidant source 202. Additionally, or alternatively, an oxidant blower or damper 238 can be operatively coupled to the controller 230 and configured to control flow of the oxidant (or combustion air).

The sensor 234 can further include a combustion sensor operatively coupled to the control circuit 230, the combustion sensor being configured to detect a temperature, video image, and/or spectral characteristic of a combustion reaction held by the perforated flame holder 102. The fuel control valve 236 can be configured to control a flow of fuel from a fuel source to the fuel and oxidant source 202. The controller 230 can be configured to control the fuel control valve 236 responsive to input from the combustion sensor 234. The controller 230 can be configured to control the fuel control valve 236 and/or the oxidant blower or damper 238 to control a preheat flame type of heater 228 to heat the perforated flame holder 102 to an operating temperature. The controller 230 can similarly control the fuel control valve 236 and/or the oxidant blower or damper 238 to change the fuel and oxidant mixture 206 flow responsive to a heat demand change received as data via the data interface 232.

FIG. 5 is a diagrammatic side-sectional view of a portion of a flare stack burner module 500 that includes a perforated flame holder 102 substantially as described with reference to FIGS. 2-3, according to an embodiment. The flare stack burner module 500 can include a housing 502 in which a flare burner 504 is positioned. The housing 502 can enclose the combustion volume 204, and includes an inlet 506, an outlet 508, and vent louvers 510.

The flare burner 504 can include the perforated flame holder 102 and a plurality of fuel nozzles 218 configured to produce fuel streams 206 directed toward respective portions of the input face 212 of the perforated flame holder 102. A fuel line 512 can extend into the housing 502 via the inlet 506, and is coupled to the plurality of fuel nozzles 218 and configured to deliver fuel to the fuel nozzles 218.

During operation, fuel, such as, for example, waste natural gas from an oil well, may be introduced via the fuel line 512 to the plurality of fuel nozzles 218, which emit respective fuel streams 206 toward the perforated flame holder 102. A combustion reaction 302 can be supported by the fuel streams 206 and held substantially within the perforations 210 of the perforated flame holder 102. Products of the combustion, such as, for example, heated air, carbon dioxide (CO₂), water vapor (H₂O), etc., exit the housing 502 via the outlet 508, whence they are dispersed in the atmosphere. Because the combustion reaction 302 is substantially contained within the perforations 210 of the perforated flame holder 102, no flames are visible outside the housing 502.

As shown in FIG. 5, the flare stack burner module 500 can be positioned at the top of a pole or stack, which serves to distribute the combustion products into the atmosphere at a height that allows them to dissipate.

Two fuel nozzles 218 are shown in the embodiment of FIG. 5. However, this is provided merely as an example. According to an embodiment (not illustrated), a flare stack burner module 500 is provided, employing a single fuel nozzle 218. According to another embodiment, a flare stack burner module 500 is provided that includes a larger number of fuel nozzles 218. For example, FIG. 7 shows a retrofit flare stack burner module 700 that includes an array of fuel nozzles 218, as described below in detail.

FIG. 6 is a diagrammatic side-sectional view of a portion of a flare stack burner module 600 that is similar in many respects to the embodiment of FIG. 5, according to an embodiment. The flare stack burner module 600 includes a flare burner 602 that includes a fuel nozzle 218 with a variable aperture 606. The fuel nozzle 218 can include a control element 604 and a nozzle outlet 610. The control element 604 can be coupled to an actuator element 608 that can be configured to move the control element 604 vertically, thereby regulating the degree to which the control element 604 occludes the nozzle outlet 610.

The size of the fuel nozzle aperture 606 may correspond to the area of the opening, as viewed in transverse section, through which the fuel stream 206 exits the fuel nozzle 218. In embodiments that include conventional fuel nozzles, the size of the fuel nozzle aperture 606 is, typically, substantially equal to the area of the corresponding opening. However, in the embodiment shown in FIG. 6, the size of the fuel nozzle aperture 606 is equal to the area of the nozzle outlet 610 minus the area of the control element 604 bisected by a plane defined by the smallest diameter of the fuel nozzle outlet 610. As the actuator element 608 moves the control element 604 upward, a larger area of the control element 604 may be bisected, reducing the size of the fuel nozzle aperture 606. Conversely, as the control element 604 is moved downward, the size of the fuel nozzle aperture 606 may increase.

In applications where the fuel supply to a flare stack may vary over time, a fixed fuel nozzle aperture may be problematic. A reduction in the fuel supply can result in a corresponding reduction in fuel stream velocity. As discussed with reference to FIG. 2, according to an embodiment, the velocity of the fuel stream 206 is preferably such that it cannot independently support a stable flame between the fuel nozzle 218 and the perforated flame holder 102. Under certain conditions, if the velocity of the fuel stream 206 is too low, a flame can begin to burn in the fuel stream 206 before it reaches the perforated flame holder 102, which would interfere with proper operation of the perforated flame holder 102, and would tend to increase undesirable emissions, such as NO_(x). In such situations, it would be desirable to increase the velocity of the fuel stream 206, in order to cause the flame to be held in the perforated flame holder 102.

It is well understood that the velocity of a fluid passing through an opening is a function of the volume of fluid passing per unit of time, and the size of the opening through which it passes. Velocity rises in direct relation to fluid volume, and in inverse relation to the opening size. Thus, with reference to the embodiment of FIG. 6, if during operation, the fuel supply drops, tending to reduce velocity of the fuel stream 206, a corresponding reduction in the size of the fuel nozzle aperture 606 will produce an increase in the fuel stream 206 velocity, and vice versa.

During operation, the fuel stream 206 may exit the fuel nozzle 218 and support a combustion reaction 302 within the perforated flame holder 102, substantially as previously described. If an increase in velocity of the fuel stream 206 is required, such as when a drop in the fuel supply to the fuel nozzle 218 causes a reduction in velocity, the actuator element 608 can be controlled to reduce the size of the fuel nozzle aperture 606, thereby increasing velocity. Likewise, where desired or required, the actuator element 608 can be controlled to increase the size of the fuel nozzle aperture 606 to reduce fuel stream 206 velocity.

According to an embodiment, the actuator element 608 is controlled by a pressure regulator feedback mechanism, in which changes in the fuel supply produce corresponding changes in fuel pressure. The pressure regulator feedback mechanism is configured to respond to these changes by increasing the size of the aperture 606 of the fuel nozzle 218 as fuel pressure increases, and by reducing the size of the aperture 606 as the fuel pressure decreases.

According to another embodiment, a controller (e.g., control circuit 230 in FIG. 2) is provided, configured to control the actuator 608 to adjust the size of the fuel nozzle aperture 606 in response to changes in one or more of fuel pressure, fuel stream 206 velocity, flame temperature, flame position, emission composition, etc.

According to a further embodiment, the actuator element 608 is configured to be controlled by an operator during operation of the flare stack burner module 600.

As previously noted, the perforated flame holder 102 is typically preheated prior to normal operation. According to another embodiment, the actuator element 608 is controlled to reduce velocity of the fuel stream 206 during a start-up procedure to permit a flame to be supported by the fuel stream 206 between the fuel nozzle 218 and the perforated flame holder 102, in order to heat the perforated flame holder 102. Once a portion of the perforated flame holder 102 reaches a selected temperature, the actuator element 608 may be controlled to reduce the fuel nozzle aperture 606 and increase the fuel stream 206 velocity, causing the flame to rise to the perforated flame holder 102.

In FIG. 6, the fuel nozzle 218 is shown as having a separate housing 612 that is configured to be coupled to a stack or pipe, and to which the housing 502 is in turn coupled. According to other embodiments, the fuel nozzle 218 is enclosed within the housing 502 or within the stack, just upstream of the housing 502.

In FIG. 6, the perforated flame holder 102 is shown occupying substantially all of the cross-sectional area of the flare stack burner module 600. According to other embodiments, as shown, for example, in FIG. 5, the perforated flame holder 102 occupies less than the entire cross-sectional area of the flare stack burner module 600. In some cases, it may be beneficial to configure a flare stack system such that no circulation of gases around the perforated flame holder 102 is permitted, while in other cases, such circulation may be advantageous. Accordingly, the determination of the size and shape of the perforated flame holder 102, in relation to the housing 502, is a design consideration.

According to an embodiment, the perforated flame holder 102 occupies between ⅔ and 100% of the cross-sectional area of the flare stack burner module 600. According to another embodiment, the perforated flame holder 102 occupies approximately ⅔ of the cross-sectional area of the flare stack burner module 600. According to a further embodiment, the perforated flame holder 102 occupies between ⅓ and ⅔ of the cross-sectional area of the flare stack burner module 600. According to an embodiment, the perforated flame holder 102 occupies the minimum cross-sectional area of the flare stack burner module 600 necessary to maintain sufficient combustion of the volatile compound. FIG. 7 is a diagrammatic side-sectional view of a portion of a flare stack burner module 700, according to an embodiment, that includes a retrofit burner 702 installed in a pre-existing flare stack. In the example shown, the pre-existing flare stack includes a fin-tube burner 704, which in turn includes a plurality of fuel tubes 706 extending substantially parallel to each other—along axes that lie perpendicular to the plane of the drawing—through transverse-oriented fin plates 708, one of which is shown. Each of the plurality of fuel tubes 706 can have a respective plurality of fuel nozzles 710 interleaved with the fin plates 708. In operation, as fuel is ejected from the fuel nozzles 710, it can entrain air passing between the fin plates 708, and a gas flare is supported inside the housing 502 and close to the fin-tube burner 704.

According to an embodiment, the retrofit flare stack burner module 702 includes a plurality of fuel nozzles 218 coupled to a common fuel line 512. The fuel nozzles 218 are interleaved between fuel tubes 706 of a start-up fin-tube burner 704. Each of the plurality of fuel nozzles 218 can be configured to provide a fuel stream 206 to a respective portion of the perforated flame holder 102. Four fuel nozzles 218 are shown in the view of FIG. 7, but the plurality of fuel nozzles 218 can include an array of fuel nozzles 218 extending beyond the plane represented in the drawing.

According to an embodiment, during a start-up procedure of the flare stack burner module 700, the fin-tube burner 704 is operated in a mode in which fuel is ejected from the fuel nozzles 710 and a flame is supported below the perforated flame holder 102, which serves to pre-heat the perforated flame holder 102. The fuel supply to the fuel tubes 706 is then cut off, and a fuel supply is supplied to the fuel line 512. Fuel streams 206 are emitted from each of the plurality of fuel nozzles 218, and a combustion reaction 302 is ignited and held in the perforated flame holder 102.

As discussed above with reference to FIG. 6, in some applications, the fuel supply can vary. Thus, according to an embodiment, valves are provided, and configured to individually control flows of fuel to each of the plurality of fuel nozzles 218. As the fuel supply increases or decreases, a corresponding number of the plurality of fuel nozzles 218 may be brought online or shut down, as necessary. According to an embodiment, the fuel supply to each of the fuel nozzles 218 is controlled so that, when additional fuel nozzles are to be brought online, only fuel nozzles that are immediately adjacent to currently operating fuel nozzles are activated. Heat from combustion supported by the adjacent fuel nozzles will enable a newly activated fuel nozzle to come up to normal operation very quickly, avoiding extended warm-up time during which unburned fuel might pass through the perforated flame holder 102.

Embodiments are described and shown in a stack configuration, i.e., a configuration in which the respective systems are supported some distance above the ground. However, other embodiments are envisioned, in which similar structures are positioned on the ground.

According to an embodiment, a natural draft low emissions combustor is contemplated to be an efficient combustion solution for oil and gas operators that can increase destruction and removal efficiency (DRE) while lowering NOx. According to an embodiment, a combustion core modular design allows for retrofits of existing combustion systems, in whole or in part. Individually and in combination, the flare stack burner modules offer large turndown ratios and are designed with multiple gas streams to accommodate various pressure streams and/or process streams.

Additionally, regulations are increasingly placing limits on low volume gas rates to minimize venting, reduce unburned hydrocarbons, and at times inefficient combustion. The flare stacks described herein can combust daily gas volumes in an extremely clean manner, all while keeping the emergency flare system in place to function as intentionally designed.

FIG. 8 is a view of a low emissions modular flare stack 800, according to an embodiment. FIG. 9 is a diagram of an arrangement 900 for each of a plurality of flare stack burner modules 802 (including, e.g., flare stack burner modules 802 a, 802 b, 802 c) of FIG. 8, according to an embodiment. As used herein, the terms flare stack burner module and burner module will be understood to be synonymous, unless context dictates to the contrary.

With reference to FIGS. 8 and 9, according to an embodiment, a low emissions modular flare stack 800 for burning condensate from petroleum production, delivery, and refining includes a plurality of flare stack burner modules 802 a, 802 b, 802 c (referenced collectively as 802). Each flare stack burner module 802 a, 802 b, 802 c may include a main fuel source 902, separately valved from all other fuel sources, configured to selectively deliver a main fuel stream 904 for dilution by a flow of combustion air. The main fuel source 902 may, in certain embodiments, correspond to or be implemented to include the volatile compound nozzle 110 described above with respect to, e.g., FIG. 1. Each flare stack burner module 802 a, 802 b, 802 c may include a main fuel igniter 906 configured to cause ignition of the main fuel stream 904 emitted from the main fuel source 902. Each flare stack burner module 802 a, 802 b, 802 c may respectively include a distal flame holder 102 configured to hold a combustion reaction supported by the main fuel stream 904 when the distal flame holder 102 is at or above a predetermined temperature. In an embodiment, the predetermined temperature may be equal to or greater than a main fuel auto-ignition temperature. Each flare stack burner module 802 a, 802 b, 802 c may include a pre-heating apparatus 908 configured to pre-heat the distal flame holder 102 to the predetermined temperature, according to an embodiment. In an embodiment, the pre-heating apparatus 908 of each flare stack burner module 802 a, 802 b, 802 c may include a continuous pilot burner that also serves as the main igniter 906. In some embodiments, the distal flame holder 102 may be separated from the main fuel source 902 and the main igniter 906 by respective non-zero distances (D1, D2).

The low emissions modular flare stack 800 may include a common combustion air source 804 configured to provide combustion air to each of the plurality of flare stack burner modules 802 a, 802 b, 802 c, and a wall 806 encircling all of the plurality of flare stack burner modules 802, the wall 806 being configured to laterally contain combustion fluids corresponding to all of the plurality of flare stack burner modules 802. In an embodiment, the distal flame holder 102 is configured to hold a combustion reaction supported by the main fuel stream 904 when the distal flame holder 102 is at or above the main fuel auto-ignition temperature. In one embodiment, each pre-heating apparatus 908 includes a continuous pilot burner. In an embodiment, the pre-heating apparatus 908 is configured selectively to output heat at any of a plurality of heating rates. At least one heating rate may be selected to cause a rise in sensible temperature of the distal flame holder 102 to the predetermined operating temperature, and at least one other heating rate may be selected to cause the pre-heating apparatus 908 to maintain a pilot flame function while a majority of total fuel consumed per unit of time is provided by the main fuel source 902. In an embodiment, the common combustion air source 804 is configured to provide natural draft combustion air to each flare stack burner module (802 a, 802 b, 802 c) of the plurality of flare stack burner modules 802.

FIG. 10 is a diagram of a control circuit 1012 for use in the control system 912 of FIG. 9, according to an embodiment.

According to an embodiment, referring to FIGS. 8, 9 and 10, the low emissions modular flare stack 800 further includes a respective plurality of separate valves 910 a, 910 b, 910 c (collectively referenced as 910), each including an actuator configured to operate responsive to receiving control signals, and further includes a control system 912 configured to output respective control signals to each of the separate valve actuators. In an embodiment, the control system 912 further includes an interface 1014 (see FIG. 10) between the control system 912 and an input channel. The input channel may include a physical (e.g., electrically conductive) connection or a wireless connection. Accordingly, the interface 1014 may include a network interface and/or a hardware interface such as, but not limited to, a USB interface, a PID controller interface, a relay interface, a radio interface, a WiFi interface, a Bluetooth interface, etc. In an embodiment, the interface 1014 includes an interface to one or more sensors disposed to sense physical parameters related to the flare stack burner modules 802 a, 802 b, 802 c, and environs. Sensors (e.g., 914 a in FIG. 9) and operation thereof, may include capacitance coupled (e.g., patch) electrodes (which may alternatively be referred to as antennas) cooperating to emit and receive a radio frequency signal across a region intended to hold a combustion reaction. A change in capacitance corresponds to a change in charged species concentration, which has been found to be covariant with the presence or absence of combustion. According to an embodiment, the electrodes may be disposed in sufficient number, and be positioned, to provide a tomographic scan of the combustion region.

The interface 1014 may be configured to receive a signal corresponding to a flare stack capacity requirement. The control system 912 may further include one or more flare stack burner module sensor inputs 1015 a, 1015 b, each of the one or more flare stack burner module sensor inputs 1015 a, 1015 b being configured to receive a signal corresponding to a flare stack burner module status, wherein the flare stack burner module status is provided by sensor hardware 914 a, 914 b, 914 c. The control system 912 may further include a microcontroller or other logic processor 1016, a computer readable memory 1018, and a module sequencer 1020 (which may optionally be embodied as or by the microcontroller 1016 and the computer readable memory 1018 when executing module sequencing functions) configured to select a subset of the plurality of flare stack burner modules 802 a, 802 b, 802 c for ignition. The control system 912 may further include a respective plurality of main fuel valve driver outputs, each operatively coupled to one of the separately actuated main fuel valves. In an embodiment, the one or more flare stack burner module sensor inputs 1015 a, 1015 b are configured to receive input from one or more sensors, such as the sensor hardware 914 a, 914 b, 914 c, or from one or more sensors external to the flare stack burner modules (802 a, 802 b, 802 c). The one or more sensors may include a demand sensor including one or more of a condensate pressure sensor, a heating energy demand sensor, and a condensate presence sensor.

In an embodiment, a control circuit 1012 (see FIG. 10) includes the module sequencer 1020. The module sequencer 1020 may include a state machine configured to changeably sequence an actuation of the plurality of flare stack burner modules 802 a, 802 b, 802 c. For example, it may be desirable to periodically change an assignment of the flare stack burner modules 802 a, 802 b, 802 c, respectively, to different positions in an actuation sequence. In one embodiment, a last module turned on in the previous module sequence may also operate as the first/only module turned on during a turn-down state. In another embodiment, the assignment of flare stack burner modules 802 a, 802 b, 802 c need not be identical with respect to capacity, age (e.g., cycle count), and design. A start-up sequence may be at least partially identical with each base demand/surge capacity cycle. The inventors contemplate various arrangements, actuation sequences, and selections of the assignment of flare stack burner modules 802 a, 802 b, 802 c may offer specific advantages to particular application characteristics.

According to an embodiment, the low emissions modular flare stack 800 further includes a run sequencer 1022. In an embodiment, the control circuit 1012 may include the run sequencer 1022. For a given module in an actuation queue served by the module sequencer 1020, the run sequencer 1022 may include a state machine configured to sequence steps in a flare stack burner module start-up schedule for one or more of the flare stack burner modules 802. Start-up schedules may be stored in the memory 1018 and periodically updated via the interface 1014 that includes a network interface. Illustrative methods and aspects for start-up sequencing are described with respect to several of the other figures included herein.

According to an embodiment, the control circuit 1012 of the low emissions modular flare stack 800 further includes an actuator driver module 1024. In an embodiment, the control circuit 1012 may include the actuator driver module 1024. The actuator driver module 1024 may be configured to provide the respective control signals to each of the separate valve actuators. The actuator driver module 1024 may include a state machine configured to load a driver shift register enable bit for amplification by a power module 1026, responsive to data from a start sequencer. Signals to/from the power module 1026 may be respectively coupled to actuatable valves 910 a, 916 a, 910 b, 916 b, 910 c, 916 c, and sensor modules 914 a, 914 b, 914 c.

Sensors 914 a, b, c, are described herein as performing sensory functions or functioning as signal outputs. In an embodiments that require outputting a signal at sensors/electrodes 914, the power module 1026 may be employed to amplify such signal, e.g., for the aforementioned emission and receipt of radio frequency signals across a combustion region. In an alternative embodiment, sensors 914 may provide a signal for generating data, e.g., a flame tomogram. In such embodiment dedicated sensor inputs 1015 a, 1015 b may be utilized. In yet another alternative embodiment, the sensors 914 a, 914 b, 914 c may be a subset of many data signals that communicate via interface 1014 of the control circuitry 1014. As described above, the interface 1014 may provide wireless or wired connections using various communication protocols, which may permit sensors 914 to communicate via a standard method such as USB, WiFi, ethernet, or the like.

According to an embodiment, the low emissions modular flare stack 800 further includes a demand module 1028. In an embodiment, demand for system capacity is received in substantially real time via a network interface included in the interface 1014. The demand module may be configured to supervise automatic operation of the flare stack burner modules selectively based on at least one of a stored schedule and a received demand signal.

The demand module 1028 may consist essentially of a data value in a register of the memory 1018. In another embodiment, especially in systems where real time data access via the interface 1014 is not guaranteed, the demand module 1028 may include a real time clock and, as data, a scheduled system capacity. In an embodiment, the demand module 1028 may operate as a supervisory state configured to automatically operate the modular flare stack 800 according to seasonal and/or periodic demand dynamics. Similarly, in an environment with chaotic dynamic demand, operation of the interface 1014 may be more crucial. In systems characterized by chaotic fluctuations in capacity demand, the inventors contemplate that an interface 104 with parallel or greater channel diversification and/or hardening may be advisable. Optionally, portions of the module sequencer 1020 may be virtualized and cloud-accessed.

According to an embodiment, the microcontroller 1016 is configured to read and execute computer executable instructions supported by a non-transitory computer readable memory 1018 to receive capacity input data corresponding to the flare stack capacity requirement signal, read data from sensor(s) corresponding to at least one flare stack burner module to verify that a selected one or more of the flare stack burner modules 802 a, 802 b, 802 c is ready for firing, select the subset of the flare stack burner modules for firing, and drive at least one of the separate main fuel valve (910 a, 910 b, 910 c) actuators corresponding to the selected subset of the plurality of flare stack burner modules (i.e., of 802 a, 802 b, and/or 802 c) to open so as to provide fuel to a combustion reaction supported by the subset of flare stack burner modules 802 a, 802 b, 802 c.

According to an embodiment, the control system 912 further includes a demand sensor. The demand sensor may include at least one of a condensate pressure sensor, a heating energy demand sensor, and/or a condensate presence sensor. In an embodiment, each flare stack burner module, e.g., 802 a, 802 b, 80 c, of the plurality of flare stack burner modules 802 further includes a pilot fuel source configured to provide a pilot fuel, a pilot fuel igniter configured to ignite a flow of the pilot fuel, and a distal pilot or start-up burner (e.g., see FIGS. 11A through 16) configured to hold a pilot flame supported by the pilot fuel, a pilot fuel source flow rate being selected to provide a pilot flame sized to raise the temperature of the distal flame holder 102 to the pre-determined temperature. In an embodiment, the predetermined temperature is equal to or greater than a main fuel auto-ignition temperature. As used herein, the terms pilot, pilot burner, distal pilot and start-up burner shall be considered synonymous unless further definition is provided.

According to an embodiment, the main igniter 906 comprises the distal pilot. According to another embodiment, the main igniter 906 includes the distal flame holder 102 when the distal flame holder 102 is heated to the pre-determined temperature by the distal pilot. That is, the fuel and combustion air may ignite on contact with the pre-heated distal flame holder 102 rather than by a separate igniter. According to an embodiment, the predetermined temperature is the main fuel auto-ignition temperature.

According to an embodiment, the distal pilot is configured to be controlled to provide the pilot flame sized to raise the distal flame holder 102 to the pre-determined temperature during a flare stack burner module start-up cycle, and to not provide the pilot flame sized to raise the distal flame holder 102 to the pre-determined temperature at times other than during the flare stack burner module start-up cycle. In one embodiment, the distal pilot is configured to decrease to a pilot flame capacity at times other than during the flare stack burner module start-up cycle. In another embodiment, the distal pilot is configured to stop supporting a combustion reaction at times other than during the flare stack burner module start-up cycle. Additionally, and/or alternatively, the distal pilot is disposed adjacent to the distal flame holder 102, and the distal pilot is controlled to be decreased to a pilot flame capacity at times other than during the flare stack burner module start-up cycle. In an embodiment, the distal pilot is configured to guarantee combustion of the main fuel, e.g., when the distal flame holder 102 does not support a combustion reaction. The main fuel may be a hydrocarbon condensate. The pilot fuel may be one or more of a hydrocarbon condensate, natural gas or propane.

According to an embodiment, a modular flare stack device includes a housing having a combustion air inlet at a base, and a plurality of flare stack burner modules positioned inside the housing. Each flare stack burner module may include an inlet configured to be coupled to a waste fuel supply and to receive the combustion air, a distal flame holder positioned distal from the waste fuel supply, and a main nozzle configured to receive a flow of main fuel from the inlet, and to emit a main fuel stream toward the distal flame holder.

According to an embodiment, each of the flare stack burner modules is configured to be freestanding, supported only by a coupling at the inlet. In another embodiment, each of the flare stack burner modules is configured to be coupled to the flare stack device and to be supported thereby.

According to an embodiment, the main nozzle is one of a plurality of main nozzles, each of the main nozzles configured to receive a flow of the main fuel from the inlet, and to emit a main fuel stream toward a respective portion of the distal flame holder.

According to an embodiment, the modular flare stack device further includes a plurality of main fuel valves operatively coupled between a common fuel line and a respective one of the plurality of main nozzles and configured to independently control operation of the respective main nozzle.

According to an embodiment, the modular flare stack device further includes a distal pilot positioned between the distal flame holder and the main nozzle for each flare stack burner module, and a retrofit burner positioned within the housing, the retrofit burner including the distal flame holder and the main nozzle. Each distal pilot may include a plurality of pilot nozzles arranged in an array. In another embodiment, the distal pilot is configured to support a pilot flame between the distal pilot and the distal flame holder. The main nozzle may include an aperture having a size that is variable. The main nozzle may be configured to regulate a velocity of the main fuel stream.

According to an embodiment, the modular flare stack device further includes an actuator operatively coupled to the main nozzle and configured to control the size of the aperture. The main nozzle may include a main nozzle outlet and a control element, the control element being positioned to occlude some portion of the main nozzle outlet, and wherein movement of the control element varies a degree to which the main nozzle outlet is occluded by the control element.

According to an embodiment, a method of using a flare stack having a plurality of flare stack burner modules includes outputting a waste gas and a supplemental fuel added to sufficiently raise a heating value of the waste gas plus the supplemental fuel to about 100 BTU per cubic foot or less toward a distal flame holder of each of the flare stack burner modules, and combusting the waste gas and the supplemental fuel substantially within one or more of the plurality of flare stack burner modules. In one embodiment, the combusting of the waste gas includes emitting a fuel stream that includes the waste gas from a main nozzle positioned within one of the flare stack burner modules of the flare stack and toward a distal flame holder of the one of the flare stack burner modules.

According to an embodiment, the method further includes, prior to performing the combusting of the waste gas, pre-heating the distal flame holder by operating a distal pilot positioned within the flare stack burner module.

According to an embodiment, the method further includes, after performing the pre-heating of the distal flame holder, shutting off a flow of pilot fuel to the distal pilot, and introducing a flow of main fuel to the main nozzle of the one of the flare stack burner modules.

According to an embodiment, the emitting the main fuel stream from the main nozzle positioned within the one of the flare stack burner modules is comprised by emitting a main fuel stream from each of a plurality of main nozzles positioned within the flare stack burner module, each toward a respective portion of the distal flame holder of the corresponding flare stack burner module. According to another embodiment, the emitting the main fuel stream from each of the plurality of main nozzles positioned within the flare stack burner module includes selecting the main nozzles of a number of the plurality of flare stack burner modules based upon a volume of waste gas to be combusted. In an embodiment, the selecting the main fuel nozzles of a number of the plurality of main nozzles of a number of the plurality of flare stack burner modules based upon the volume of waste gas to be combusted comprises varying the number of the plurality of flare stack burner modules in response to changes in the volume of waste gas.

According to an embodiment, the method further includes venting products of the combustion to the atmosphere.

FIG. 11A is a block diagram of a flare stack burner module 1100 for a flare stack, according to an embodiment. The flare stack burner module 1100 for a flare stack includes a pilot fuel nozzle 1104 and a plurality of main fuel nozzles 1106. The pilot fuel nozzle 1104 and the main fuel nozzle 1106 are disposed in a flare stack burner module volume 1101.

According to an embodiment, the pilot fuel nozzle 1104 is disposed in a flare stack burner module at a distal position along a fuel flow axis A. The plurality of main fuel nozzle(s) 1106 are disposed at a proximal position along the fuel flow axis A.

According to an embodiment, the pilot fuel nozzle 1104 is configured to support a pilot flame 1108. As is described in more detail below, the pilot flame 1108 helps to ignite and/or sustain a main flame 1110.

In one embodiment, the pilot fuel nozzle 1104 supports the pilot flame 1108 by outputting a pilot fuel 1112. The pilot flame 1108 is supported by the pilot fuel 1112 and combustion air introduced into the flare stack burner module volume 1101. Accordingly, the pilot flame 1108 is a combustion reaction of the pilot fuel 1112 and combustion air. In one embodiment, the pilot flame 1108 is a primary flame.

In one embodiment, the main fuel nozzles 1106 are configured to support the main flame 1110 within the flare stack burner module volume 1101. The main flame 1110 is supported downstream from the pilot flame 1108.

In one embodiment, the main fuel nozzles 1106 support the main flame 1110 by outputting a main fuel 1114 into the flare stack burner module volume 1101. The main flame 1110 is supported by the main fuel 1114 and combustion air that is introduced into the flare stack burner module volume 1101. Accordingly, the main flame 1110 is a reaction of the main fuel 1114 and the combustion air.

In one embodiment, the pilot fuel nozzle 1104 is configured to support a pilot flame 1108 and the plurality of main fuel nozzles 1106 are configured to simultaneously support the main flame 1110 in contact with the pilot flame 1108, the main flame 1110, at least once, being ignited by the pilot flame 1108.

In one embodiment, the main flame 1110 may not be in contact with the pilot flame 1108. Instead, the main flame 1110 may be separated from the pilot flame 1108 by a gap.

In one embodiment (e.g., as in FIG. 12A, described below), the pilot fuel nozzle 1104 includes a plurality of tubes or arms that extend laterally from the fuel and combustion air flow axis A. The tubes or arms include a plurality of orifices that output the pilot fuel 1112. Accordingly, the pilot flame 1108 is held above each of the tubes, arms, or segments of the pilot fuel nozzle 1104. The shape of the pilot fuel nozzle 1104 can be selected to cover a lateral area corresponding to the area above the main fuel nozzles 1106, or an area through which the main fuel 1114 passes.

In one embodiment, the main fuel 1114 passes through gaps more open spaces between the laterally extending portions of the pilot fuel nozzle 1104. The main fuel 1114 may be initially ignited by the pilot flame 1108 as the main fuel 1114 passes adjacent to the pilot flame 1108. After the main fuel 1114 has been ignited, thereby generating the main flame 1110, the main flame 1110 can be supported in a steady state by the main fuel 1114.

In one embodiment, the pilot fuel nozzle 1104 is a pilot fuel manifold. The pilot fuel manifold includes laterally extending tubes, segments, arms, or portions. The pilot fuel 1112 is output from orifices positioned in the laterally extending tubes, segments, arms, or portions of the pilot fuel manifold.

According to an embodiment, the main flame 1110 includes a flame having a heat output of at least 10 times the heat output of the pilot flame 1108 when the flare stack burner module 1100 is operating at a rated heat output. In one embodiment, operating the flare stack burner module 1100 at the rated heat output corresponds to operating in a steady state standard operating mode of the flare stack burner module 1100. In another embodiment, the main flame 1110 includes a flame having a heat output of at least 20 times the heat output of the pilot flame 1108 when the flare stack burner module 1100 is operating at a rated heat output.

According to an embodiment, the flare stack burner module 1100 has a NOx output of about twenty parts per million or less, adjusted to 3% excess O₂ at a stack operatively coupled to the flare stack burner module 1100.

FIG. 11B is a block diagram of a flare stack burner module 1111 including a bluff body flame holder, or distal flame holder, 1102, according to an embodiment. The flare stack burner module 1111 of FIG. 11B is substantially similar to the flare stack burner module 1100 of FIG. 11A, except that the flare stack burner module 1111 includes a bluff body flame holder 1102.

According to an embodiment, the flare stack burner module 1111 includes a plurality of main fuel main fuel nozzles 1106 disposed at a proximal position along a flow axis A of a flare stack burner module volume 1101, a pilot fuel nozzle 1104 disposed at an intermediate distance along the flow axis A, and a distal flame holder 1102 disposed at a distal position along the flow axis A. The pilot fuel nozzle 1104 may be configured to support a pilot flame 1108 to heat the distal flame holder 1102. The main fuel nozzle(s) 1106 may be configured to provide main fuel 1114 to the distal flame holder 1102 after the distal flame holder 1102 is at least partially heated. The distal flame holder 1102 may be configured to hold at least a portion of the main combustion reaction 1110 supported by the main fuel 1114.

In one embodiment, the bluff body flame holder 1102 is a perforated flame holder. The operation and structure of the perforated flame holder are described with more particularity in relation to FIGS. 2-4 and 17A-17B.

FIG. 12A is an illustration of a flare stack burner module 1200, according to an embodiment. The flare stack burner module 1200 includes a pilot fuel nozzle 1104 and main fuel nozzles 1106. Though not shown in FIG. 12A, the pilot fuel nozzle 1104 is configured to support the pilot flame 1108 by outputting the pilot fuel 1112. Although not shown in FIG. 12A, the main fuel nozzles 1106 are configured to support the main flame 1110 by outputting the main fuel 1114.

According to an embodiment, the pilot fuel nozzle 1104 is supported by and receives fuel via a pilot fuel pipe 1220 configured to support the pilot fuel nozzle 1104 at the distal position. The pilot fuel pipe 1220 extends into the flare stack burner module volume 1101 via an opening 1240 in a bottom 1238 of the flare stack burner module 1200. An outer support stiffener 1222 can be positioned around the pilot fuel pipe 1220 and configured to substantially prevent the pilot fuel pipe 1220 from wobbling. The main fuel nozzles 1106 also extend through the opening 1240 in the bottom 1238. The main fuel nozzles 1106 can be supported by fuel risers 1224. In one embodiment, the main fuel nozzles 1106 include orifices that output the main fuel 1114 with a 2° spread.

According to an embodiment, the pilot fuel nozzle 1104 defines a plurality of fuel orifices 1218 having a large collective area to collectively support a low momentum pilot flame 1108. In an embodiment, the main fuel 1114, output by the main fuel nozzles 1106, and combustion air form a combustible mixture that expands in breadth as it flows from the proximal position of the main fuel nozzles 1106 to the distal position of the pilot fuel nozzle 1104. The plurality of fuel orifices 1218 may be disposed across the flare stack burner module volume 1101 sufficiently broadly to cause contact of the pilot flame (1108) with the main fuel (1114) and combustion air mixture across the breadth of the combustible mixture. In another embodiment, the main fuel nozzles 1106 are configured to output fuel in co-flow with the air.

According to an embodiment, the pilot fuel nozzle 1104 includes a fuel manifold having a plurality of segments 1219 joined together, each segment 1219 having a plurality of fuel orifices 1218 configured to pass fuel from inside the fuel manifold to the flare stack burner module volume 1101. The plurality of segments 1219 may be formed as respective tubes configured to freely pass the fuel delivered from the pilot fuel pipe 1220 into the fuel manifold. In one embodiment, at least a portion of the tubes is arranged as spokes radiating from a center disposed substantially at a centerline along the axis. In another embodiment, at least a portion of the tubes is arranged as an “ ”, a rectangle, an “H”, a wagon wheel, or a star.

According to an embodiment, the pilot fuel nozzle 1104 includes a manifold including a curvilinear tube. In an embodiment, the curvilinear tube is arranged as a spiral, “

”, “

”, or “∞”.

According to an embodiment, the main fuel nozzles 1106 form a first fuel dump plane at a proximal location, e.g., coincident with or near the bottom 1238 of the flare stack burner module 1200.

According to an embodiment, the pilot fuel nozzle 1104 forms a second fuel dump plane at the distal location at least 100 main fuel nozzle 1106 diameters from the bottom 1238 of the flare stack burner module 1200.

According to an embodiment, the pilot fuel nozzle 1104 includes at least one tube disposed transverse to the fuel and combustion air flow axis A. The at least one tube may include opposed vertical tabs extending upward from the at least one tube to form a “U” channel.

According to an embodiment, the flare stack burner module 1200 further includes a distal flame holder disposed at a third position along the fuel and combustion air flow axis A, more distal from the main fuel nozzles 1106 than the pilot fuel nozzle 1104. In an embodiment, the bluff body flame holder includes a perforated flame holder 1102 configured to control a flame length. The perforated flame holder 1102 can hold at least a portion of the main flame 1110 within the perforated flame holder 1102.

According to an embodiment, the flare stack burner module 1200 includes a pilot fuel source 1230. The pilot fuel source 1230 supplies the pilot fuel 1112 into the pilot fuel pipe 1220. The pilot fuel 1112 is output from the pilot fuel nozzle 1104 via the fuel orifices 1218. A pilot fuel control valve 1234 can be manually or electronically controlled to enable or shut off the flow of pilot fuel 1112 from the pilot fuel source 1230 to the pilot fuel nozzle 1104.

According to an embodiment, the flare stack burner module 1200 includes a main fuel source 1232. The main fuel source 1232 supplies the main fuel (1114) to the main fuel nozzles 1106. The main fuel (1114) can be supplied to the main fuel nozzles 1106 via the fuel risers 1224. A main fuel control valve 1236 can be manually or electronically controlled to enable or shut off the flow of the main fuel (1114) from the main fuel source 1232 to the main fuel nozzles 1106.

In one embodiment, the combustion air is drafted or input into the flare stack burner module volume 1101 through the opening 1240 in the bottom 1238 of the flare stack burner module 1200. Additionally, or alternatively, the combustion air can be provided into the flare stack burner module volume 1101 in ways other than through the opening 1240 in the bottom 1238.

FIG. 12B is an illustration of a flare stack burner module 1211, according to an embodiment. The flare stack burner module 1211 is substantially similar to the flare stack burner module 1200 of FIG. 12A, except that the flare stack burner module 1211 includes a distal flame holder 1102 positioned above the pilot fuel nozzle 1104. While the pilot fuel nozzle 1104 supports the pilot flame 1108 (see FIG. 11A and FIG. 11B), the distal flame holder 1102 holds the main flame 1110 (see FIG. 11B). The pilot flame 1108 can ignite and stabilize the main flame 1110.

According to an embodiment, the distal flame holder 1102 includes a perforated flame holder configured to hold the main flame 1110. The perforated flame holder 1102 can hold at least a portion of the main flame 1110 within the perforated flame holder 1102. In another embodiment, the distal flame holder 1102 may include one or more solid bluff bodies (aka “tiles”) configured to hold a main flame adjacent to the surface of the solid bluff bodies. In another embodiment, the distal flame holder 1102 may include a mixture of perforated tiles and solid tiles configured to maintain ignition of combustion fluids in proximity to the tiles.

FIG. 13 is a perspective view of a flare stack burner module 1300, according to an embodiment. The flare stack burner module 1300 includes a pilot fuel nozzle 1104 and main fuel nozzles 1106. Although not shown in FIG. 13, the pilot fuel nozzle 1104 is support the pilot flame 1108 by outputting the pilot fuel 1112. Though not shown in FIG. 13, the main fuel nozzles 1106 are configured to support the main flame 1110 by outputting the main fuel 1114.

According to an embodiment, the pilot fuel nozzle 1104 is supported by and receives the pilot fuel (1112) via a fuel pipe 1220. The fuel pipe 1220 extends into the flare stack burner module volume 1101 via an opening (1240) in a bottom 1238 of the flare stack burner module 1300. A stiffener 1222 can be positioned around the fuel pipe 1220 to prevent the fuel pipe 1220 from wobbling. The main fuel nozzles 1106 also extend through the opening (1240) in the bottom 1238. The main fuel nozzles 1106 can be supported by fuel risers 1224. In one embodiment, the main fuel nozzles 1106 include orifices that output the main fuel (1114) with less than a 15° spread.

According to an embodiment, the pilot fuel nozzle 1104 includes a fuel manifold having a plurality of segments 1219 joined together, each segment 1219 having a plurality of fuel orifices 1218 configured to pass fuel from inside the fuel manifold to the flare stack burner module volume 1101. The plurality of segments 1219 may be formed as respective tubes configured to freely pass the fuel delivered from the pilot fuel pipe 1220 into the fuel manifold. In one embodiment, at least a portion of the tubes is arranged as spokes radiating from a center disposed substantially at a centerline along the axis A. In the embodiment of FIG. 13, the plurality of segments 1219 are arranged in an “ ” shape.

In an embodiment, each segment 1219 includes one or more sections of reticulated ceramic 1226 disposed in and supported by a “U” channel in the segments 1219. The pilot fuel 1112 can flow from the fuel orifices 1218 into the channels or passageways of the one or more sections of reticulated ceramic 1226. The pilot flame 1108 can be held at least partially within the channels or passageways of the one or more sections of reticulated ceramic 1226.

In one embodiment, the one or more sections of reticulated ceramic 1226 are disposed superjacent to the at least one tube. The at least one tube may define a plurality of pilot fuel flow apertures disposed along a length of the at least one tube. In an embodiment, the at least one tube defines a plurality of fuel flow apertures configured to allow gaseous pilot fuel (1112) to flow upward into a “U” channel formed superjacent to the at least one tube.

In one embodiment, the flare stack burner module 1300 includes support legs 1252. The support legs 1252 can support a distal flame holder 1102 in the flare stack burner module volume 1101 above the pilot fuel nozzle 1104. The distal flame holder 1102 can hold a portion of the main flame 1110.

FIG. 14 is an illustration of a pilot fuel nozzle 1104 in the shape of an H, according to an embodiment. The pilot fuel nozzle 1104 includes a plurality of fuel orifices 1218 that can output the pilot fuel (1112). The pilot fuel nozzle 1104 can be made up of a plurality of tubes segment joined together to form the “H” shape.

FIG. 15 is an illustration of a pilot fuel nozzle 1104 in the shape of a spiral, according to an embodiment. The pilot fuel nozzle 1104 includes a plurality of fuel orifices 1218 that can output the pilot fuel (1112). The pilot fuel nozzle 1104 can be made up of a plurality of tubes segment joined together to form the spiral shape.

FIG. 16 is an illustration of a pilot fuel nozzle 1104 in the shape of a hexagon with sides attached to a center hub, according to an embodiment. The pilot fuel nozzle 1104 includes a plurality of fuel orifices 1218 that can output the pilot fuel (1112). The pilot fuel nozzle 1104 can be made up of a plurality of tube segments joined together to form the shape shown in FIG. 16.

FIG. 17A is a simplified perspective view of a combustion system 1700, including an alternative distal flame holder 1702, according to an embodiment. The distal flame holder 1702 is a reticulated ceramic flame holder, according to an embodiment. The distal flame holder 1702 may include a plurality of reticulated fibers. FIG. 17B is a simplified side sectional diagram of a portion of the reticulated ceramic distal flame holder 1702 of FIG. 17A, according to an embodiment. The distal flame holder 1702 of FIGS. 17A, 17B can be implemented in the various combustion systems described herein, according to an embodiment. The distal flame holder 1702 is configured to support a combustion reaction (e.g., combustion reaction 302 of FIG. 3) of the main fuel and oxidant mixture 1706 received from the fuel and oxidant source 1703 at least partially within the perforated flame holder 1702. According to an embodiment, the distal flame holder 1702 can be configured to support a combustion reaction of the main fuel and oxidant mixture 1706 upstream, downstream, within, and adjacent to the reticulated ceramic perforated flame holder 1702.

According to an embodiment, the perforated flame holder body 1708 can include reticulated fibers 1739. The reticulated fibers 1739 can define branching perforations 1710 that weave around and through the reticulated fibers 1739. According to an embodiment, the perforations 1710 are formed as passages between the reticulated fibers 1739.

According to an embodiment, the reticulated fibers 1739 are formed as a reticulated ceramic foam. According to an embodiment, the reticulated fibers 1739 are formed using a reticulated polymer foam as a template. According to an embodiment, the reticulated fibers 1739 can include alumina silicate. According to an embodiment, the reticulated fibers 1739 can be formed from extruded mullite or cordierite. According to an embodiment, the reticulated fibers 1739 can include Zirconia. According to an embodiment, the reticulated fibers 1739 can include silicon carbide.

The term “reticulated fibers” refers to a netlike structure. According to an embodiment, the reticulated fibers 1739 are formed from an extruded ceramic material. In reticulated fiber 1739 embodiments, the interaction between the main fuel and oxidant mixture 1706, the combustion reaction (302), and heat transfer to and from the perforated flame holder body 1708 can function similarly to the embodiment shown and described above with respect to FIGS. 2-4. One difference in activity is a mixing between “perforations” 1710 compared with the perforations 210 in FIGS. 2-3, because the reticulated fibers 1739 form a discontinuous perforated flame holder body 1708 that allows flow back and forth between neighboring perforations 1710.

According to an embodiment, the network of reticulated fibers 1739 is sufficiently open for downstream reticulated fibers 1739 to emit radiation for receipt by upstream reticulated fibers 1739 for the purpose of heating the upstream reticulated fibers 1739 sufficiently to maintain combustion of a main fuel and oxidant mixture 1706. Compared to a continuous perforated flame holder body 208, heat conduction paths (such as heat conduction paths 312 in FIG. 3) between reticulated fibers 1739 are reduced due to separation of the reticulated fibers 1739. This may cause relatively more heat to be transferred from a heat-receiving region or area (such as heat receiving region 306 in FIG. 3) to a heat-output region or area (such as heat-output region 310 of FIG. 3) of the reticulated fibers 1739 via thermal radiation (shown as element 304 in FIG. 3).

According to an embodiment, individual perforations 1710 may extend between an input face 1712 to an output face 1714 of the perforated flame holder 1702. Perforations 1710 may have varying lengths L. According to an embodiment, because the perforations 1710 branch into and out of each other, individual perforations 1710 are not clearly defined by a length L.

According to an embodiment, the perforated flame holder 1702 is configured to support or hold a combustion reaction (see element 302 of FIG. 3) or a flame at least partially between the input face 1712 and the output face 1714. According to an embodiment, the input face 1712 corresponds to a surface of the perforated flame holder 1702 proximal to the main fuel nozzle 1718 or to a surface that first receives fuel. According to an embodiment, the input face 1712 corresponds to an extent of the reticulated fibers 1739 proximal to the main fuel nozzle 218. According to an embodiment, the output face 1714 corresponds to a surface distal to the main fuel nozzle 1718 or opposite the input face 1712. According to an embodiment, the input face 1712 corresponds to an extent of the reticulated fibers 1739 distal to the main fuel nozzle 1718 or opposite to the input face 1712.

According to an embodiment, the formation of thermal boundary layers 314, transfer of heat between the perforated flame holder body 1708 and the gases flowing through the perforations 1710, a characteristic perforation width dimension D, and the length L can each be regarded as related to an average or overall path through the perforated reaction holder 1702. In other words, the dimension D can be determined as a root-mean-square of individual Dn values determined at each point along a flow path. Similarly, the length L can be a length that includes length contributed by tortuosity of the flow path, which may be somewhat longer than a straight-line distance T_(RH) from the input face (e.g., 1712) to the output face (1714) through the perforated reaction holder 1702. According to an embodiment, the void fraction (expressed as (total perforated reaction holder volume—reticulated fiber volume)/total volume)) is about 70%.

According to an embodiment, the reticulated ceramic perforated flame holder 1702 is a tile about 1″×4″×4″. According to an embodiment, the reticulated ceramic perforated flame holder 1702 includes about 100 pores per square inch of surface area. Other materials and dimensions can also be used for a reticulated ceramic perforated flame holder 1702 in accordance with principles of the present disclosure.

According to an embodiment, the reticulated ceramic perforated flame holder 1702 can include shapes and dimensions other than those described herein. For example, the perforated flame holder 1702 can include reticulated ceramic tiles that are larger or smaller than the dimensions set forth above. Additionally, the reticulated ceramic perforated flame holder 1702 can include shapes other than generally cuboid shapes.

According to an embodiment, the reticulated ceramic perforated flame holder 1702 can include multiple reticulated ceramic tiles. The multiple reticulated ceramic tiles can be joined together such that each ceramic tile is in direct contact with one or more adjacent reticulated ceramic tiles. The multiple reticulated ceramic tiles can collectively form a single perforated flame holder 1702. Alternatively, each reticulated ceramic tile can be considered a distinct perforated flame holder 1702.

FIG. 18 is a flow chart showing a method 1800 for operating a flare stack burner module (e.g., 802) that includes a pilot flame holder (e.g., 908, 1104) and a distal flame holder (e.g., 102, 1102, 1702) when the distal flame holder includes a perforated flame holder, according to an embodiment. According to an embodiment, the method 1800 includes, in step 1802, providing heat to the distal flame holder with the pilot flame holder, the distal flame holder and the pilot flame holder being disposed in the flare stack burner module and in proximity to one another. Step 1804 includes introducing main fuel and combustion air to the pilot flame holder and the distal flame holder. Step 1806 includes holding at least a portion of a combustion reaction of the main fuel and combustion air at the distal flame holder while the pilot flame remains lit.

FIG. 19 is a flow chart showing a method 1900 for operating a flare stack burner module, according to an embodiment. According to an embodiment, the method 1900 includes, in step 1902, supporting a diffusion flame across a portion of a width of a flare stack burner module volume at a position distal from a flare stack burner module bottom. Step 1904 includes providing combustion air to the flare stack burner module volume from a location near the flare stack burner module bottom. Step 1906 includes outputting high pressure main fuel jets from a plurality of main fuel nozzles at the location near the flare stack burner module bottom. Step 1908 includes mixing the main fuel with the combustion air while the main fuel and combustion air travels from the location near the flare stack burner module bottom to the distal position of the distal flame holder. Step 1910 includes combusting the main fuel by exposing the mixed main fuel and air to the diffusion flame and the heated distal flame holder.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

1. A low emissions modular flare stack, comprising: a plurality of flare stack burner modules, each flare stack burner module including: a main fuel source, separately valved from all other fuel sources, configured to selectively deliver a stream of a main fuel for dilution by a flow of combustion air, a main fuel igniter configured to cause ignition of the main fuel emitted from the main fuel source, a distal flame holder, separated from the main fuel source and the main fuel igniter by respective non-zero distances, the distal flame holder being configured to hold a combustion reaction supported by the main fuel when the distal flame holder is at or above a predetermined temperature, and a pre-heating apparatus configured to pre-heat the distal flame holder to the predetermined temperature; a common combustion air source configured to provide combustion air to each of the plurality of flare stack burner modules; and a wall encircling all of the plurality of flare stack burner modules, the wall being configured to laterally contain combustion fluids.
 2. The low emissions modular flare stack of claim 1, wherein the predetermined temperature is equal to or greater than a main fuel auto-ignition temperature.
 3. The low emissions modular flare stack of claim 1, wherein the pre-heating apparatus of each flare stack burner module comprises a continuous pilot burner configured to guarantee combustion of the main fuel.
 4. The low emissions modular flare stack of claim 3, wherein the continuous pilot burner is configured to selectively output heat at any of a plurality of heating rates, at least one heating rate is selected to cause a rise in sensible temperature of the distal flame holder to the predetermined operating temperature; and at least one other heating rate is selected to cause the pre-heating apparatus to maintain a pilot flame function while a majority of total fuel consumed per unit time is provided by the main fuel source.
 5. (canceled)
 6. The low emissions modular flare stack of claim 1, wherein the common combustion air source is configured to provide natural draft combustion air to each flare stack burner module of the plurality of flare stack burner modules.
 7. The low emissions modular flare stack of claim 1, further comprising: a respective plurality of separate valves, each including an actuator configured to operate responsive to receiving control signals; and a control system configured to output respective control signals to each of the separate valve actuators; wherein the control system further comprises: an interface between the control system and an input channel, wherein the interface is configured to receive a signal corresponding to a flare stack capacity requirement; one or more flare stack burner module sensor inputs, each of the one or more flare stack burner module sensor inputs being configured to receive a signal corresponding to a flare stack burner module status wherein the flare stack burner module status is provided by sensor hardware; a microcontroller, a computer readable memory, and a module sequencer configured to select a subset of the plurality of flare stack burner modules for ignition; and a respective plurality of main fuel valve driver outputs each operatively coupled to one of the separately actuated main fuel valves.
 8. The low emissions modular flare stack of claim 7, wherein the control system further comprises one or more of: a module sequencer configured to changeably sequence an actuation of the flare stack burner modules; a run sequencer including a state machine configured to sequence steps in a start-up schedule of the flare stack burner modules; an actuator driver module configured to provide the respective control signals to each of the separate valve actuators; and a demand module configured to supervise automatic operation of the flare stack burner modules selectively based on at least one of a stored schedule and a received demand signal. 9.-11. (canceled)
 12. The low emissions modular flare stack of claim 7, wherein the microcontroller is configured to read and execute computer executable instructions, supported by a non-transitory computer readable memory, to: receive capacity input data corresponding to the flare stack capacity requirement signal; read module status sensor data corresponding to at least one flare stack to verify that a selected one of the flare stack burner modules is ready for firing; select the subset of the plurality of flare stack burner modules for firing; and drive at least one of the separate valve actuators corresponding to the subset of the selected plurality of flare stack burner modules to open so as to provide fuel to a combustion reaction supported by the subset of the plurality of flare stack burner modules. 13.-14. (canceled)
 15. The low emissions modular flare stack of claim 1, wherein each of the plurality of flare stack burner modules further comprises: a pilot fuel source configured to provide a pilot fuel to the pre-heating apparatus, and a pilot fuel igniter configured to ignite a flow of the pilot fuel, and wherein the pre-heating apparatus includes a distal pilot configured to hold a pilot flame supported by the pilot fuel, a pilot fuel source flow rate being selected to provide a pilot flame sized to raise the distal flame holder temperature to the pre-determined temperature.
 16. (canceled)
 17. The low emissions modular flare stack of claim 15, wherein the main fuel igniter comprises the distal pilot.
 18. The low emissions modular flare stack of claim 15, wherein the main fuel igniter comprises the distal flame holder when the distal flame holder is heated to the pre-determined temperature by the distal pilot.
 19. The low emissions modular flare stack of claim 15, wherein the distal pilot is configured to be controlled to provide the pilot flame sized to raise the distal flame holder to the pre-determined temperature during a flare stack burner module start-up cycle, and to not provide the pilot flame sized to raise the distal flame holder to the pre-determined temperature at times other than during the flare stack burner module start-up cycle. 20.-23. (canceled)
 24. The low emissions modular flare stack of claim 15, wherein at least one of the main fuel and the pilot fuel is a hydrocarbon condensate.
 25. (canceled)
 26. The low emissions modular flare stack of claim 15, wherein the pilot fuel is natural gas or propane.
 27. The low emissions modular flare stack of claim 1, wherein: the wall constitutes at least part of a housing having a combustion air inlet at a base; the main fuel source includes an inlet configured to be coupled to a waste fuel supply; the common combustion air source is positioned to receive the combustion air via the housing; the distal flame holder is positioned inside the housing; and the main fuel source includes a main nozzle configured to receive a flow of at least the waste fuel as a main fuel from the inlet, and to emit Hall the main fuel stream toward the distal flame holder.
 28. The low emissions modular flare stack of claim 27, wherein each of the flare stack burner modules is configured to be freestanding, supported only by a coupling at the inlet.
 29. The low emissions modular flare stack of claim 27, wherein each of the flare stack burner modules is configured to be coupled to the flare stack wall and to be supported thereby.
 30. The low emissions modular flare stack of claim 27, wherein the main nozzle is one of a plurality of main nozzles respectively corresponding to each flare stack burner module, each of the main nozzles being configured to receive a flow of the main fuel from the inlet, and to respectively emit the main fuel stream toward a respective portion of the distal flame holder.
 31. The low emissions modular flare stack of claim 30, further comprising a plurality of main fuel valves operatively coupled between a common fuel line and a respective one of the plurality of main nozzles and configured to independently control operation of the respective main nozzle.
 32. (canceled)
 33. The low emissions modular flare stack of claim 3, wherein each continuous pilot includes a plurality of pilot fuel nozzles arranged in an array.
 34. (canceled)
 35. The low emissions modular flare stack of claim 27, wherein the main nozzle includes an aperture having a size that is variable.
 36. The low emissions modular flare stack of claim 35, wherein the main nozzle is configured to regulate a velocity of the main fuel stream. 37.-39. (canceled)
 40. A method of using a flare stack having a plurality of flare stack burner modules, the method comprising: outputting, toward a distal flame holder of each flare stack module of the plurality of flare stack burner modules, a waste gas and a supplemental fuel added to sufficiently raise a heating value of the waste gas plus the supplemental fuel to about 100 BTU per cubic foot or less; and combusting the waste gas and the supplemental fuel substantially within one or more of the plurality of flare stack burner modules.
 41. The method of claim 40, wherein the combusting of the waste gas comprises emitting a fuel stream that includes the waste gas from a main nozzle positioned within one of the flare stack burner modules of the flare stack and toward a distal flame holder of the one of the flare stack burner modules.
 42. The method of claim 41, further comprising, prior to performing the combusting of the waste gas, pre-heating the distal flame holder by operating a distal pilot positioned within the flare stack burner module.
 43. The method of claim 42, further comprising, after performing the pre-heating of the distal flame holder: shutting off a flow of pilot fuel to the distal pilot; and introducing a flow of main fuel to the main nozzle of the one of the flare stack burner modules.
 44. The method of claim 41, wherein the emitting the main fuel stream from the main nozzle positioned within the one of the flare stack burner modules is comprised by emitting a main fuel stream from each of a plurality of main nozzles positioned within the flare stack burner module, each toward a respective portion of the distal flame holder of the corresponding flare stack burner module.
 45. The method of claim 44, wherein the emitting the main fuel stream from each of the plurality of main nozzles positioned within the flare stack burner module comprises selecting the main nozzles of a number of the plurality of flare stack burner modules based upon a volume of waste gas to be combusted.
 46. The method of claim 45, wherein the selecting the main fuel nozzles of the number of the plurality of main nozzles of a number of the plurality of flare stack burner modules based upon the volume of waste gas to be combusted comprises varying the number of the plurality of flare stack burner modules in response to changes in the volume of waste gas. 47.-56. (canceled)
 57. The low emissions modular flare stack of claim 15, wherein the distal pilot includes one or more pilot fuel nozzles, each pilot fuel nozzle defining a plurality of fuel orifices have a large collective area to collectively support a low momentum pilot flame.
 58. (canceled)
 59. The low emissions modular flare stack of claim 57, wherein the plurality of fuel orifices are disposed across the flare stack burner module volume sufficiently broadly to cause contact of the pilot flame with the main fuel and combustion air mixture across the breadth of the combustible mixture.
 60. (canceled)
 61. The low emissions modular flare stack of claim 15, wherein the distal pilot includes a single pilot fuel nozzle comprising a fuel manifold having a plurality of segments joined together, each segment having a plurality of fuel orifices configured to pass fuel from inside the fuel manifold to a flare stack burner module combustion volume.
 62. The low emissions modular flare stack of claim 61, wherein the plurality of segments are formed as respective tubes configured to freely pass the pilot fuel delivered from a pilot fuel pipe into the fuel manifold.
 63. (canceled)
 64. The low emissions modular flare stack of claim 27, wherein the pre-heating apparatus forms a second fuel dump plane at a distal location at least 100 main fuel nozzle diameters from a bottom of the respective flare stack burner module of the plurality of flare stack burner modules. 65.-68. (canceled)
 69. The low emissions modular flare stack of claim 1, further comprising support legs supporting the distal flame holder in each flare stack burner module.
 70. (canceled)
 71. A method for operating a flare stack burner module having a distal flame holder, comprising: providing heat to the distal flame holder with a pilot flame holder, the distal flame holder and the pilot flame holder being disposed in the flare stack burner module and in proximity to one another; introducing main fuel and combustion air to the pilot flame holder and the distal flame holder; and holding at least a portion of a combustion reaction of the main fuel and combustion air at the distal flame holder while the pilot flame holder remains lit.
 72. (canceled) 